I had a chance to test iPhone 4 and iPhone 4s side-by-side at AT&T store in California. Verdict:
+ Slightly better camera with video stabilization
+ Slightly faster
+ Bluetooth 4.0
- Less standby time
- Oldschool design
- More expensive
Everything else is just iOS 5.
But I think the camera is worth it, so I’ll be still getting one, as I’ve been waiting for a year to replace my 3GS.