without any USB devices attached except for my mouse and/or keyboard, the suspend does work.
After i attach other USB devices it looks like XBMC is getting into trouble. When i hit suspend it wakes up immediately or it does crash before the suspend.
fritsch helped me out finding the problem and brought the solution.
USB TV Tuner: Anysee E30 C
case "$1" in
service tvheadend stop
service tvheadend start
sudo chmod +x /etc/pm/sleep.d/99-restart-tvheadend.sh
sudo reboot (with the adapter plugged in)