After installing 10.6 SL on my iMac, Macbook Pro and Mac Book, my external Firewire Pioneer burner in a 3rd party enclosure failed. Our local MUG BBS came up with an easy fix.
Access a backup from 10.5 or another Mac running 10.5 and open SYSTEM and then EXTENSIONS. Copy from your 10.5 Extensions
Attach your copy device and replace the 10.6 extension with the 10.5 extension. It will require authorization to do so. You MUST repair permissions BEFORE YOU RESTART and then go ahead and restart. Once restarted, you should be good to go.
Note: I had to do this twice with the second effort working. I suspect Apple should be notified of this so it can be remedied in 10.6.2, but am not sure how to effectively get this information to someone who will pay attention.