I got one of the last new N900s from Nokia, after about 2-3 years though even this gem is starting to show wear. The most troublesome scratches were to the lens of the main camera which had become nearly useless producing only unfocused washed out pictures.
I did some research and found a guide for fixing a N95's camera with Displex display polish in a red tube.
http://www.allaboutsymbian.com/featu...mera_glass.php
I figured it was safest to go with what that guy used to fix what is almost the exact same camera as the N900, and can confirm that after I polished almost all of the red anti-glare coaring off of the lens I have gone from taking hazy useless photos back to normal shots though perhaps with a bit more blue balance in low light.
I also polished where the anti-scratch coating has cracked off of the digitizer and it smoothed it up enough to stick properly to a new screen cover.

The instructions are all in French which I do not read well.
I put a drop of paste onto a cotton swab and polished the camera for a few minutes, after which I wiped the lens with a damp cotton washcloth several times, then with a dry cotton cloth.
This was enough to return function to the camera as well as polish the digitizer.

I have seen people swapping out the lens cover plastic, but this fix is simple and I have seen no downside.