The lens is made from sapphire which is extremely scratch resistant. It has a hardness of 9 out of 10 and only diamonds and a couple other substances can scratch it so I wouldn't worry at all about scratching the lens.
I was caseless with my iPhone 4 and quite careful with it. The camera lens DID end up being really scratched up though.. I dont know if they changed the material for the camera lens since then but if not, you can expect scratches...