ACPI support in linux and debian is improving but it is not as simple or easy as the other commerical OS. Below you will find links to more information on ACPI.
http://www.columbia.edu/~ariel/acpi/acpi_howto.0.2c.html - Up to date documentation (Oct 2005).
http://acpi.sourceforge.net/ - A page created by Intel developers working on patching the kernel to improve ACPI support.
http://www.tldp.org/HOWTO/ACPI-HOWTO/ - Some older info.
In Debian Synaptic Package Manager you can search on ACPI and get a list of tools that help you monitor and initiate.
My Sarge install didn't include acpid, and hunting around for /proc/acpi/battery/BATn proved fruitless, until I installed acpid. Woohoo! aptitude update && aptitude upgrade && aptitude install acpid, and there it all was. Re-start Gkrellm, and there's its builtin displaying the battery state (you need to first turn the builtin on, of course). Thanks for that columbia.edu link!