The Collaborative International Dictionary of English v.0.48:

Obsolescent \Ob`so*les"cent\, a. [L. obsolescens, -entis, p. pr.
   of obsolescere, to wear out gradually, to fall into disuse;
   ob (see Ob-) + solere to use, be wont.]
   Going out of use; becoming obsolete; passing into desuetude.
   [1913 Webster]

WordNet (r) 3.0 (2006):

obsolescent
    adj 1: becoming obsolete