Live Chess! Live Chess! Live Chess! 
Computer Workout would be good too.
I'd vote for Chess Mentor, but I have yet to get enough cash to get the diamond account so I can use it.
I use Tactics Trainer and correspondence chess on my iPhone already. It's not too bad in mobile Safari. Except I can't type comments / forum posts with mobile Safari, but that's an entirely different issue I suppose.
).
Would this be a local engine on the device, or all server-side and requiring connectivity? I don't really consider this feature one of chess.com's core competencies or real differentiators on the web, and there is some *stiff* competition in the field of apps already. I guess it would add value for folks who will never install a second chess app anyway.
we're starting work on the chess.com iphone app. let's discuss features here!
the 3 things i would like to include are:
- play against your friends (correspondence-style)
- tactics trainer
- play computer
thoughts?