Before forking out for a new back cover, I though I'd try getting rid of the cheap plastic covering. I cracked it and peeled it off. It's just glued on to the black inner ring. Easy. You almost can't tell the difference - just a hole where the cheap clear plastic was. The picture quality is hugely improved. Be interesting to do this with a new cover - try video with and without the backcover. I'm confident the lens itself is protected enough.
Got the idea from
HTC Desire Tricks: How to Improve Your Camera (picture and video) Quality | Mobile Phone Blog