Hi RPD,
Useful kernel panic, something between our process and the USB framework. Or inside the framework itself. But no reproduction for us so far.
We have seen in our lab a system freeze that was as mysterious, simply having any event on the USB bus while in sleep caused it but only on one specific machine.
In that case it helped to resetting the NVRAM to default.
I would try to reset the the NVRAM, the instructions are in
http://support.apple.com/kb/ht1379
Carlo