Good news: I have my N900, and the cheap eBay batteries work. They are, as expected, worth exactly as much as I paid for them.

I'll post lshal results and use observations in the next day or two. For now, know that they report a design capacity of ~600 mAh. They hold this charge exceptionally well - after 5 hours of heavy use, still 100% charge. Can't beat that kind of staying power!

Summary: these work, but have less capacity than the OEM and have non-functional onboard controllers, so state of charge (SoC) and voltage are not correctly reported to the N900. For emergency/backup use, I'm fine with this, but I won't be using them under normal circumstances.

Has anyone else purchased cheap batteries that correctly report SoC?