Thread about this already
http://talk.maemo.org/showthread.php?t=56706&page=2

oddly I wasn't having this problem before (as evident in that thread) but it did start happening at some point. What is really odd though is that I have had my N900 since launch yet people had this problem before mine started. Why would that happen seeing as they could only have had it the same length of time as me?

I think this is a bug when you reflash. Look at that post and this it's exactly the same; after a reflash this starts to occur. My phone has since been reflashed too which might be when I started to see this. I don't think it's the button battery that has ran out.