I think it's going to be iPhone 4S. The iPhone 4 was a huge leap in both design and hardware. I really don't see that happening again this year.
One thing I would bet my money on is that we will
not see a design change. Did you see Steves face when he unveiled iPhone 4? He is in love with it. It will obviously change but not this year! Maybe a bit smaller/larger but that's it.
Just my opinion of course...