I don't know about Germany, but in the United States the warranty procedure was pretty straightforward. I called them and told them the problems I was having, I sent them just the phone (battery cover, SD card, etc is not needed), and they sent it back with new parts installed. They paid for shipping both ways so it cost me nothing other than driving to drop off the package.

Besides, if Nokia's new batch of N900s have a new keyboard and trim installed from the factory as a previous post seems to indicate, then it would occur to me that Nokia is aware of the problem being there from the beginning and should fix it for free.