I am having the same exact problem! I knew this would be current! This is such a turn off =( Nokia CS told me the same thing but the problem is that I'm in Canada and they said they don't repair this phone in Canada... this is frustrating... I can't even find a replacement cover in ebay...

People will start to experience this problem soon! My advice would be:

-Whenever you slide the camera cover, do it slowly and don't put pressure on it.
-Try not to put it too often in your pockets cause everyti e you slide your n900 in your pocket, you can accidently slide the camera cover or break it.
-Always put your N900 on a flat area.

I have kinda fixed mine with crazy glue. I glued the corners of the open area. mine was opened in the bottom right. Before fixing this, everytime I slide it, there was a tiny metel thing that slips out and it didn't close properly... Now glued, it works but it doesn't feel right but it closes and opens perfectly without he slingy feeling.