The Collaborative International Dictionary of English v.0.48:

Unattainable \Unattainable\
   See attainable.

WordNet (r) 3.0 (2006):

unattainable
    adj 1: impossible to achieve; "an unattainable goal" [syn:
           unachievable, unattainable, undoable,
           unrealizable]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "unattainable":
   beyond one, beyond reach, closed forever to, closed to,
   impenetrable, impervious, impracticable, impractical, inaccessible,
   infeasible, inoperable, insuperable, insurmountable, lost to,
   out of reach, too much for, un-come-at-able, unaccessible,
   unachievable, unapproachable, unavailable, uncompassable,
   undiscoverable, undoable, uneffectible, unfeasible, ungetatable,
   ungettable, unnegotiable, unobtainable, unperformable,
   unprocurable, unreachable, unrealizable, unsecurable,
   unsurmountable, unworkable