I had the same problem that ended up with me loosing the top buttons altogether. I just found them and it took me about an hour to get the positioning correct to get it back on.
Even though it stays on now it's still quite loose.
I would suggest getting in there with a pencil and moving the little connecting pieces to the correct position and trying to click it back in before it gets worse like mine. I wish I could show you pictures but there are little pegs that connect to the outside ring (zoom piece) (the pegs are underneath the ring) and all you have to do is line them up.
Sidenote: if you don't understand what I mean either try googling pictures or just send it in. Might be safer to send it in if it's still under warranty.
NameGoesHere, January 2009