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.
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   2. Delicately constituted or constructed; nice; fine;
      delicate; tenuous; finely woven. "A sotil [subtile]
      twine's thread." --Chaucer.
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            More subtile web Arachne can not spin. --Spenser.
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            I do distinguish plain
            Each subtile line of her immortal face. --Sir J.
                                                  Davies.
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   3. Acute; piercing; searching.
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            The slow disease and subtile pain.    --Prior.
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   5. Characterized by nicety of discrimination; discerning;
      delicate; refined; subtle. [In this sense now commonly
      written subtle.]
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            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.
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            The subtile influence of an intellect like
            Emerson's.                            --Hawthorne.
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   5. Sly; artful; cunning; crafty; subtle; as, a subtile
      person; a subtile adversary; a subtile scheme. [In this
      sense now commonly written subtle.]
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   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.
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Moby Thesaurus II by Grady Ward, 1.0:

96 Moby Thesaurus words for "subtile":
   Byzantine, Machiavellian, Machiavellic, acute, adulterated,
   air-built, airy, arch, artful, astute, attenuate, attenuated,
   cagey, calculating, canny, chimerical, clever, cloud-built, crafty,
   cunning, cut, cute, dainty, deceitful, deep, deep-laid, delicate,
   designing, devious, dilute, diluted, diplomatic, dreamlike,
   ethereal, fanciful, fatuitous, fatuous, feline, fine, flimsy, foxy,
   gaseous, gossamery, guileful, illusory, imaginary, ingenious,
   insidious, insubstantial, inventive, knowing, pawky, phantomlike,
   politic, rare, rarefied, ready, resourceful, scheming, serpentine,
   shadowy, sharp, shifty, shrewd, slick, slight, slippery, sly,
   smooth, snaky, sneaky, sophistical, spirituous, stealthy,
   strategic, subtle, supple, tactical, tenuous, thin, thinned,
   thinned-out, trickish, tricksy, tricky, uncompact, uncompressed,
   unreal, unsubstantial, vaporous, vulpine, wary, watered,
   watered-down, wily, windy