Warranty probably wont cover your lens cover as it can be caused by harsh use. You should just find new back cover and pay little for it. There was some website somewhere where they sell all n900 parts and back cover shouldn't be that expensive.

That's in finnish but ordering shouldn't be that hard: http://www.klc.fi/fin/tuotteet/Nokia-Varaosat-N900

Then i found this: http://www.thenokiaparts.com/eng/tuo...00-spare-parts
Dont know how it differs from klc as they have different stocks but same prices.

15€ for back cover ain't so bad. I paid 45€ for new lcd screen =)