The Collaborative International Dictionary of English v.0.48:

Unobservant \Unobservant\
   See observant.

WordNet (r) 3.0 (2006):

unobservant
    adj 1: not consciously observing; "looked through him with blank
           unseeing eyes" [syn: unobservant, unseeing]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "unobservant":
   careless, disregardant, disregardful, distracted, distrait,
   heedless, inadvertent, inattentive, inconstant, incurious,
   indifferent, inobservant, negligent, nonadherent, noncompliant,
   nonconforming, nonobservant, regardless, thoughtless, uncompliant,
   unconforming, unfaithful, unheedful, unheeding, unloyal, unmarking,
   unmindful, unnoticing, unnoting, unobserving, unperceiving,
   unremarking, untrue, unwatchful