I was rather impressed by:

http://www.eastmaze.com/High-Capacit...1394.html?c=63

In a few test runs it's come out as 1310 - 1330 mAh using N900's own battery fuel gauge chip, and assuming RS=21 as determined experimentally by one of the kind sould in #meego.

Impressive that it's as good or better than original Nokia BL-5J, which iirc tested out as 1250-something with the same method.

Internal resistance "feels" like its lower, but that might just be a sign that there's no protection circuit in it...

The bq27200 fuel gauge doesn't use any method close to a "proper" one. It just integrates current discharged, until it reaches 3248mV (and the point at which this happens varies with system load, of course), and then assumes that the battery is at 6%, calibrates charge meter according that. The 0% is assumed to be at 3V.
Under normal operations N900 doesn't reach voltages this low.

My 2400mAh mugen showed up as 2400mAh with the same method when new, and now after some months of abuse it shows up as 2300mAh.