The Collaborative International Dictionary of English v.0.48:

Unimportant \Unimportant\
   See important.

WordNet (r) 3.0 (2006):

unimportant
    adj 1: not important; "a relatively unimportant feature of the
           system"; "the question seems unimportant" [ant:
           important, of import]
    2: devoid of importance, meaning, or force [syn:
       insignificant, unimportant] [ant: important,
       significant]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "unimportant":
   casual, homely, humble, humble-looking, humble-visaged, humblest,
   inconsiderable, inferior, inglorious, innocuous, insignificant,
   least, light, low, low-priority, lowest, lowliest, lowly, marginal,
   mean, minor, modest, no great shakes, of no account,
   of no consequence, of no matter, of no significance, petty, plain,
   poor, secondary, shoestring, simple, small, small-beer, teachable,
   trivial, undistinguished, unpretentious