[Hampshire] Intercepting shutdown

Does anyone know if there's a way to intercept, or hook into, gnome's
shutdown procedure?
Basically, my computer has a tv card and occasionally I forget it's
recording the tv and shutdown. So what I'd like is to get gnome to call
some script of mine to run some checks and tell me not to shutdown.
Failing that is there a way I can do this with run-levels?

Thanks,
Leo