For destructive users, i would highly recommend silicone cases or leather cases. iSkin cases for iphone are durable for it has shock and impact protection. Check out iSkin for iPhone 4 is coming!
I am currently using iphone 4 skins from sourcingmap. They are cool, cheap and give my phone more protection from scratches, dust without adding any bulk. but i don't think they are the most durable cases, they are just thin covers.