I have separate cables I use at home and work, and I have a few here at home that I've tried. They all work with my "test" N900 fine, so I'm happy to rule out the cables and PCs. It's either hardware on my N900 or software on my N900. I doubt the chances of corrupting not only NOLO's USB mode, but bootmenu's modules, AND the actual phone's proper modules as well.

...which just leave the hardware. I wonder if it could be a joint that has continuity but is starting to fail, making the resistance too high or the circuit too flaky for USB signalling to pass?