I've done some (non-scientific) research and believe he has the dreaded plantar fasciitis. Reports indicate he can walk and run straightline but can't cut. He also has a history of foot problems, including most recently at the end of last season in December, 2005.
Link: http://fantasyfootball.usatoday.com/conten...2552&line=68028.
Plantar fasciitis is a painful and slow-healing injury in which the thin soft tissue in the arch of the foot closer to the heel is inflamed. It makes lateral movement (i.e., a wide receiver's cuts) very difficult. The injury is such that it appears to "get better" followed by setbacks due to inflammaton. This is consistent with the reports coming out of the Bengals (i.e., he's playing, he's not playing, etc.)