The Collaborative International Dictionary of English v.0.48:

Guileful \Guile"ful\, a.
   Full of guile; characterized by cunning, deceit, or
   treachery; guilty. -- Guile"ful*ly, adv. --
   Guile"ful*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

guileful
    adj 1: marked by skill in deception; "cunning men often pass for
           wise"; "deep political machinations"; "a foxy scheme"; "a
           slick evasive answer"; "sly as a fox"; "tricky Dick"; "a
           wily old attorney" [syn: crafty, cunning, dodgy,
           foxy, guileful, knavish, slick, sly, tricksy,
           tricky, wily]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "guileful":
   Byzantine, Machiavellian, Machiavellic, acute, arch, artful,
   astute, cagey, calculating, canny, chiseling, clever, collusive,
   covinous, crafty, cunning, cute, deceitful, deep, deep-laid,
   designing, devious, diplomatic, duplicitous, false, falsehearted,
   feline, finagling, foxy, fraudulent, furtive, indirect, ingenious,
   insidious, inventive, knowing, pawky, politic, ready, resourceful,
   scheming, serpentine, sharp, shifty, shrewd, slick, slippery, sly,
   smooth, snaky, sneaking, sneaky, sophistical, stealthy, strategic,
   subtile, subtle, supple, surreptitious, tactical, treacherous,
   trickish, tricksy, tricky, two-faced, underhand, underhanded,
   vulpine, wary, wily