This problem appeared when I updated my BIOS to its latest version to solve another issue. This happens not very often so the computer is still usable most of the time, but probably there is a problem with the BIOS or something, because I am not the only one with issues with NVIDIA + ipw3945 on a Pavilion system:
I have updated to 2.6.22-gentoo-r8 and will test this (I have to wait some time to see if something happens...). Meanwhile I will try to document myself for the IRQ managment stuff and will wait for answeres on the ipw3945 bug tracker (it's possible that the problem is on their side).
Thanks again for your help.