Those are hardware limitations, not software......
All iOS4 features will be available....but there are some things that will be on iPhone 4, that are not on the 3G(s) such as the gyroscope and front facing camera
just the same as 2G...no GPS, no 3G connectivity
just the same as the 3G...no compass, no video recording
now, just the same as the 3G(s)...no front facing camera or gyroscope
then, just the same as the iPhone 4...just kidding