Interestingly, I recently had a 3-week trip to Japan, and I took my N900 for SMSes, and borrowed my brother's old HTC Desire so I could have a decent Google Maps experience using a data-only SIM.

Anyway, because of this issue, I had to charge my N900 using a power-only USB cable, and things were fine whilst I was away.

When I returned however, I needed to download my photos, some of which were on my N900. I figured I'd try plugging it in via USB as a random act of desperation...and my PC picked it up. It works normally again!

Tried it at work, and same...it picks it up and I can sync my calendar again

I'm a happy bunny. I've no idea how whatever was broken righted itself other than either the battery pack charger I was using managed to "unlatch" whatever minor internal fault was disrupting the USB data lines with higher current (the charger is a 2A one), or the tighter connector somehow improved the contacts on the USB plug? The always had continuity...but maybe the contacts weren't good enough for data? Who knows. All I care is that it works again