The Collaborative International Dictionary of English v.0.48:

Subtile \Sub"tile\, a. [L. subtilis. See Subtile.]
   1. Thin; not dense or gross; rare; as, subtile air; subtile
      vapor; a subtile medium.
      [1913 Webster]

   2. Delicately constituted or constructed; nice; fine;
      delicate; tenuous; finely woven. "A sotil [subtile]
      twine's thread." --Chaucer.
      [1913 Webster]

            More subtile web Arachne can not spin. --Spenser.
      [1913 Webster]

            I do distinguish plain
            Each subtile line of her immortal face. --Sir J.
                                                  Davies.
      [1913 Webster]

   3. Acute; piercing; searching.
      [1913 Webster]

            The slow disease and subtile pain.    --Prior.
      [1913 Webster]

   5. Characterized by nicety of discrimination; discerning;
      delicate; refined; subtle. [In this sense now commonly
      written subtle.]
      [1913 Webster]

            The genius of the Spanish people is exquisitely
            subtile, without being at all acute; hence there is
            so much humor and so little wit in their literature.
            The genius of the Italians, on the contrary, is
            acute, profound, and sensual, but not subtile; hence
            what they think to be humorous, is merely witty.
                                                  --Coleridge.
      [1913 Webster]

            The subtile influence of an intellect like
            Emerson's.                            --Hawthorne.
      [1913 Webster]

   5. Sly; artful; cunning; crafty; subtle; as, a subtile
      person; a subtile adversary; a subtile scheme. [In this
      sense now commonly written subtle.]
      [1913 Webster]

   Syn: Subtile, Acute.

   Usage: In acute the image is that of a needle's point; in
          subtile that of a thread spun out to fineness. The
          acute intellect pierces to its aim; the subtile (or
          subtle) intellect winds its way through obstacles.
          [1913 Webster] -- Sub"tile*ly, adv. --
          Sub"tile*ness, n.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Subtle \Sub"tle\, a. [Compar. Subtler; superl. Subtlest.]
   [OE. sotil, subtil, OF. soutil, later subtil, F. subtil, L.
   subtilis; probably, originally, woven fine, and fr. sub under
   + tela a web, fr. texere to weave. See Text, and cf.
   Subtile.]
   1. Sly in design; artful; cunning; insinuating; subtile; --
      applied to persons; as, a subtle foe. "A subtle traitor."
      --Shak.
      [1913 Webster]

   2. Cunningly devised; crafty; treacherous; as, a subtle
      stratagem.
      [1913 Webster]

   3. Characterized by refinement and niceness in drawing
      distinctions; nicely discriminating; -- said of persons;
      as, a subtle logician; refined; tenuous; sinuous;
      insinuating; hence, penetrative or pervasive; -- said of
      the mind; its faculties, or its operations; as, a subtle
      intellect; a subtle imagination; a subtle process of
      thought; also, difficult of apprehension; elusive.
      [1913 Webster]

            Things remote from use, obscure and subtle.
                                                  --Milton.
      [1913 Webster]

   4. Smooth and deceptive. [Obs.]
      [1913 Webster]

            Like to a bowl upon a subtle ground [bowling
            ground].                              --Shak.
      [1913 Webster]

   Syn: Artful; crafty; cunning; shrewd; sly; wily.

   Usage: Subtle is the most comprehensive of these epithets and
          implies the finest intellectual quality. See Shrewd,
          and Cunning.
          [1913 Webster]

WordNet (r) 3.0 (2006):

subtle
    adj 1: difficult to detect or grasp by the mind or analyze; "his
           whole attitude had undergone a subtle change"; "a subtle
           difference"; "that elusive thing the soul" [syn:
           elusive, subtle]
    2: able to make fine distinctions; "a subtle mind"
    3: working or spreading in a hidden and usually injurious way;
       "glaucoma is an insidious disease"; "a subtle poison" [syn:
       insidious, pernicious, subtle]

Moby Thesaurus II by Grady Ward, 1.0:

288 Moby Thesaurus words for "subtle":
   Byzantine, Machiavellian, Machiavellic, abstruse, accurate, acute,
   adulterated, air-built, airy, analytic, analytical, appreciative,
   arcane, arch, artful, astute, attentive, attenuate, attenuated,
   balled up, boyish, cagey, calculating, canny, casuistic,
   chimerical, civilized, clever, close, cloud-built, complex,
   complicated, concealed, confounded, confused, conscientious,
   constant, convoluted, correct, crabbed, crafty, creamy, critical,
   cultivated, cultured, cunning, cut, cute, daedal, dainty,
   deceitful, deceptive, deep, deep-laid, delicate, demanding,
   designing, detailed, devious, dexterous, diaphanous, differential,
   dilute, diluted, diplomatic, direct, discriminate, discriminating,
   discriminative, distinctive, distinguishing, dreamlike, eggshell,
   elaborate, elegant, elusive, embrangled, entangled, ethereal, even,
   exact, exacting, exigent, express, exquisite, faint, faithful,
   fanciful, fastidious, fatuitous, fatuous, feline, fine, fine-drawn,
   finespun, finical, finicking, finicky, flat, flimsy, fouled up,
   foxy, frail, fussy, gaseous, gauzy, girlish, gloss, gossamer,
   gossamery, graceful, gracile, gracious, guileful, hairline,
   hairsplitting, hidden, illusory, imaginary, implicated, inerrable,
   inerrant, infallible, ingenious, insidious, insubstantial,
   intricate, inventive, involuted, involved, iridescent, knotted,
   knowing, labyrinthian, labyrinthine, lacy, light, loused up,
   many-faceted, mathematical, matted, mazy, meandering, mellow,
   messed up, meticulous, micrometrically precise, microscopic,
   minute, misty, mixed up, mother-of-pearl, mucked up, multifarious,
   nacreous, narrow, nebulous, nice, obscure, opalescent, pale,
   papery, particular, pastel, patinaed, pawky, pearly, perplexed,
   phantomlike, pinpoint, polished, politic, precise, precisianistic,
   precisionistic, profound, punctilious, punctual, quiet, ramified,
   rare, rarefied, ratiocinative, ready, recondite, refined,
   religious, religiously exact, remote, resourceful, rigid, rigorous,
   roundabout, sad, scheming, scientific, scientifically exact,
   screwed up, scrupulous, scrutinizing, selective, semigloss,
   sensitive, serpentine, severe, shadowy, sharp, shifty, shrewd,
   simple, skillful, slender, slenderish, slick, slight, slight-made,
   slim, slimmish, slimy, slinky, slippery, sly, small, smarmy, smart,
   smooth, snaky, snarled, sneaky, sober, soft, soft-colored,
   soft-hued, softened, somber, sophistical, sophisticated,
   spirituous, square, stealthy, strategic, strict, subdued, subtile,
   supple, svelte, sweet, sylphlike, tactful, tactical, tangled,
   tangly, tender, tenuous, thin, thin-bodied, thin-set, thin-spun,
   thinned, thinned-out, thinnish, threadlike, trickish, tricksy,
   tricky, twisted, uncompact, uncompressed, underhanded, undeviating,
   unerring, unreal, unsubstantial, vague, vaporous, veiled, vulpine,
   wary, wasp-waisted, watered, watered-down, watery, weak, willowy,
   wily, windy, wiredrawn, wispy