This happened to me yesterday, on an up-to-date N900. I didn't notice the battery going flat (my old E71 would scream at me :-) and when I came to plug it in, I got a solid orange light.

I have left it plugged in to the mains charger overnight, while I read up round here, with similar issues. I have attempted to get it in to R&D mode, using flasher-3.5 (I have the Mac OS X version) but flasher tells me there is no USB device found, and sure enough, the phone does not show up in the USB device tree in System Profiler. Now, this could be because I haven't quite got the sequence of events to get the thing into USB mode right. Can someone clarify? And help me out here?!

Cheers,
James