My Nokia N810 has a BP-4L battery with 1500 mAh.The new Nokia N900 will get the BL-5J with only 1320 mAh.
That is 180 mAh less in a successor.
Strange because the N900 has now things like:

-Integrated FM transmitter
-5 megapixel camera with dual led flash
-Call and data features like 3G and HSDPA.

And much more that the N810 did not have , so the N900 will need more power but gets less.
The N900's 1320 mAh battery has 12% (180mAh) less then the N810's 1500 mAh battery.

But there is one thing more to tell about the battery of the n900 , it is only a Li-Ion battery against the much better Li-Polymer of the N810.
When compared to a lithium-ion battery, a Li-poly has a greater life cycle , and lower weight and greatly increased run times.

Nokia gives the N900 not only less battery-power , from 1500mAh to "only" 1320 mAh but also a worse battery-technologie from Li-Polymer to Li-Ion.