The offensive ROY is really a tight race. Caddy still deserves consideration, as does Heath if he can continue to produce, but Ronnie Brown and Kyle Orton have been excellent in pivotal roles too, and should compete.
I think defensively the ROY is much easier to determine. Odell Thurman. He's easily become a cornerstone of that Bengals defense, and with Madieu Williams out he is easily their best defensive player. Just look at the stats:
35 solo, 13 assists, 1 sack, 4 INTs, 1 for a TD, 3 forced fumbles, and 6 passes defended.
Derrick Johnson still has a chance to compete, but he either needs to outdistance himself in tackles by a lot, or come up with a few more big plays, otherwise this is Odell's ROY.