I recently installed FreeBSD on my IBM ThinkPad X20 laptop, it seemes
ok, I never tried it before - but I got most of the things I wanted up
and running.

I still do have a major concern: When shutting down the computer,
either with 'halt' and the power button or with 'shutdown -p now',
after some minutes the computer powers itself on again. This is
becomming a major pain in the rear since the computer powers on in my
backpack home from work, and when I'm done with dinner the battery is
flat.

Hope some of you gurus out there can help me, I'm still a freshman in
the FreeBSD class.

Greetings
Arne

comp.unix.bsd.freebsd is not widely propagated, so you won't
necessarily get much help here (in comp.unix.bsd.freebsd).
See http://www.freebsd.org/support.html for better ideas...
[followups redirected accordingly]

That kind of functionality is usually driven by the BIOS.
You should probably start by checking whatever "wake-up"
functionality is turned on there...