I also had this problem. I don't know if it's a bug in gnome-power-manager or in the kernel, but I think the changes between versions 2.6.38.4 and 2.6.38.5 of battery.c in the Linux kernel triggered this behaviour. I'm now using kernel 2.6.39, but with the version of battery.c from 2.6.38.4 and it's fine.
