There is a pretty long discussion in http://community.active.com/thread/152667/after-the-c25k-continuing-the-journey/0/0 you might want to check out.
The most common next step seems to be Bridge to 10k (B210K) but some people are doing the "become a one hour runner" thing and some are even going straight to the half marathon training plans.
Started C25K 4/14/11, finished 6/23/11
Allen Stone 5k 7/16/11 (32:59)
Started B210k 7/20/11, finished 8/31/11
McDonald's Richmond Half-Marathon 11/12/11 (2:05:29)
Shamrock Half-Marathon 3/18/12 (2:09:43)
Monument Ave 10k 3/31/12 (no goal -- just hoping I'm still walking after the half)