Read this:
This 42-watt-hour rechargeable lithium ion battery provides up to 5 hours of operation depending on battery saving features used and system configuration. This battery does not work with previous models of iBook which have 2 battery latches. This battery is compatible only with new opaque-white, 12.1" iBooks having two USB ports, CD-ROM, and 700MHz PPC G3 processor.
Does this mean this batteries don't work with the 500, 600 series. I can't see why not, but you never know.
On the apple websites, it doesn't say that, but on the box to the battery at the apple store it says something similar to the above.