This sounds like a "don't do that then" but it happens in real situations with
networkmanager

So the simple reproducer is:

 - bring up eth0 which is a wireless airo
 - "rmmod airo"
 - hangs with "unregister_netdevice: waiting for usage count to be 0"
 - all network-related processes are now locked up, so you can't 
   do anything to bring the usage count to 0
 - you can't kill any of the network-related processes
 - you have to reboot afaict

If you do the rmmod after an "ifdown eth0" then it works fine.

The reason this happens with NetworkManager is basically:
 - across suspend/resume the airo driver gets confused out of its mind
 - without NetworkManager you can work around this by "ifdown eth0; rmmod airo; 
   ifup eth0"
 - with NetworkManager you can't do the "ifdown" part of the workaround so it 
   would be convenient if "rmmod airo" worked with the interface still up, 
   or "rmmod airo; killall NetworkManager" worked

There are probably better fixes, like:
 - have the airo driver not get confused when you suspend
 - have NetworkManager bring down the interface on suspend

Anyway, something is hosed, feel free to bounce this buck around ;-)

Thanks