The Power-Manager shows power supply on battery, when the cabel is unplugged. The bug is since kernel 3.0.6 (a function, which was signed as deprecated in 3.0.4, is deleted in 3.0.6). Systeminformation: Arch Linux x86_64 (3.0.6-ARCH)
Not sure, maybe I misunderstand something here, but isn't that exactly the expected behavior to show that the system is "on battery" when the ac cable is unplugged? Also, if this is a bug in the kernel, it's not a bug in xfce4-power-manager.
Closing.