I played bike-baron on 3gs and on 4s. There are real-time shadows on the later. Is this a possibility we can use on 4s too?
You should be able to get it on iPhone 4 (and maybe even on 3Gs) as well! I don’t see why you can’t do it?!
You just need a proper shader to handle shadowing…
Stencil shadows wouldn’t be so hard to implement, specially if you use one model for rendering and one low-quality version of it for the shadow 
I believe the problem right now, is that unity does not give you access to the stencil buffer. Also, shadows happen on every object in the game, not just the player. Hopefully unity will give us access to real time shadows on uneven terrain in the future. If you are looking to put shadows on flat terrain, however, there is a script on the unity wiki that will allow you to do that.