I don't use a cover.
I honestly can't see the point in messing up a nicely designed phone with some cheap, ugly covering. I doubt most of the covers are going to do much more than protect the phone from anything more than scratches, and what's the point of that? ...so you can have a scratch free phone sitting on your shelf when you upgrade to the latest model the following year. Cool.
In the end, to me it's just a cellphone. You don't see people with their blackberries/nokia etc all covered up like it's their precious newborn baby, what makes the iPhone so special? Just look after it like you would any other phone and it'll last