So I wasn't able to go to the nokia shop but I found somebody who wanted to do it for me with the following result:

A brand spanking new back cover for my N900. The guy at nokia said that it was communicated out of nokia that it was a known issue.

The funny thing was that the guy at the nokia store had to start the phone to reset the settings for the new back cover. But all the settings are still like it set them up. It was the first n900 they got back so I guess they hadn't had enough of n900 playtime.