I think the package just needs to be rebuilt. Forgive me for not trying this on debian directly, but after apt-getting the xfce4-battery-plugin source and building/installing it in debug mode on ubuntu hardy the battery meter worked. I then rebuilt it without debug enabled and it continued to work.
To be honest I have no idea what's happening but I am not sure that it is a kernel problem in any obvious way since acpi reports correct information all along and hal seems sane. Can anyone confirm that this actually happens on debian proper? I am wondering if everyone here are ubuntu users who have been referred upstream. Kind Regards, Alfred Rossi