Funnily enough, the very same thing happened to me just yesterday.

I can't say exactly what did it (though it was almost certainly my fault - I'd been fiddling with apps from extras-* all day) but even so, I had exactly the symptoms you describe. My battery wasn't low, but after rebooting the device voluntarily I had a reboot loop.

I charged up my Nokia *fully* - better to be safe than sorry - and followed the instructions on the Wiki:

http://wiki.maemo.org/Updating_the_tablet_firmware

A few (stressful!) minutes later I had a working N900 again, and an hour of installing later all my apps etc. back in place.