i've got the same problem as well, and couldnt find a solution for it either (from a previous thread - cant seem to find that thread now though). so i'm stuck without it.. i imported the N900 from the US and i live in Australia, so no chance of me claiming warranty here
someone suggested pressing the left part of the keyboard (near the Ctrl or Function keys) and that has worked. maybe you can give that a shot?
Dunno if it helps, but the n900 powers down the transmitter when charging. You need to use FM-boost to boost it back up again once charging has started.