I bumped into this one on Mugen's site:
1320mAh replacement for BL-5J

Now, according to this article, on N97 Mugen Power's 1800mAh replacement greatly increased battery life compared to the original 1500mAh while fitting in with the old back cover. So maybe this one is worth considering for the N900, even though it is quite pricey.

The original N900 battery is 1320mAh anyway, so you won't get any increase of battery life. Pricewise it looks about the same as a genuine Nokia battery, so I wouldn't bother.

It's not just the mAh printed on the battery that matters. As you can see there's only 300mAh (20%) increase on the N97 battery yet the battery life is increased by a LOT more.

It's on the way. I will report how's the battery life but I guess it will be at least 4-5 days. I have like four extended batteries for different devices from Mugen already. They're all great.

im just worried about the new back, possibly making camera not function correctly.. for e.g. you cannot use flashlight app without back on

I really need that huge battery, my N900 lasts 6 hours after charge with moderate usage.. (Only GSM + Google talk, no wi-fi, 3G, GPS etc, 2 desktop pages one with icons and one with 3 links + media player widget)

With the new cover, N900 detects it as cover missing. Camera still works, but flashlight doesn't, it probably wants the cover to be in place and camera door opened?

What about the memory card - does that still work?

Nope, memory card doesn't work.

The back cover is detected using a magnetic field sensor - no magnet, no back cover, no memory card.

The state of the camera cover is detected using some type of light sensor - as the door opens iit slides a (highly reflective?) white patch in front of a sensor. When this patch is removed the door is considered closed.

I guess the Mugen battery has the white patch, so the device thinks the door is permanently open, but it has no magnet, so it never knows the case is attached! Not sure why the flashlight doesn't work in this situation, though.

I suppose you could add your own magnet by gluing part of a fridge magnet inside the Mugen cover, which should work - there must be plenty of room! Although it may end up being too far away from the sensor to be detected, so gluing it permanently to the magnet sensor may be the only option.

Can't help thinking Mugen have done a terrible job on this product - the battery is way too big for ordinary people, and the cover they replaced wasn't just a simple hunk of plastic.