I just found out that the most ideal case for protecting the iPhone in a slim form factor is the Clarifi from Griffin, However, It's only available for the 3g/3gs iPhone.
When you say "lens cover" do you mean an actual protective barrier that COVERS the lens? Or do you just mean a case that protects AROUND the camera/flash area?
I've been using an otterbox or the most part and they provide a good amount of protection. The lens doesn't have anything specifically covering it, but with the design. Other then directly nothing can get in.