I have a similar (but perhaps not identical) problem. The last couple of days it seems that plugging my N900 into the wall charger hasn't worked properly, and tonight I caught it in the act! It started charging for maybe 5 minutes and then stopped, sending me a message along the lines of "unplug charger to save electricity" and then wouldn't start charging again no matter how many times I removed the battery and SIM card etc etc.

It's off to the store tomorrow to see what they suggest, but I'm not hugely optimistic at this point