It shouldn't affect the picture quality of the phone in the least. I've seen cameras covered with dust still take half-way decent pictures. In my experience with owning several Env models (V to Touch) with similar lens styles, you'd be hard pressed to actually scratch your lens to a point where it will make a difference anyway.
The answer might be to get a phone case to at least somewhat protect your lens. This won't protect direct poking but it should at least provide a slight height difference so that keys, cards, whathaveyou won't run a huge scratch across the back of your phone and the lens. The other solution is to purchase a pouch or holster that completely covers the back side of the phone, but of course then you'd have to take the phone out of it every time to use it.
The ultimate answer really is just make sure that your droid is the only thing in your pocket, and if you are a girl who carries a purse, make sure the droid isn't in it.
On a side note, I wouldn't worry too much about photo quality as our phones are quite lackluster in that department unfortunately. Oh well can't win em all. =)
