I have the same problem.

My ATA DVD writer is recognized by Leopard and it works fine (all the DVDs I burned work).

On Snow Leo, however, it doesn't even show in Disk Utility and if I insert a DVD in the drive nothing happens. It's as if I don't have a DVD drive.

My chipset is ICH9 (this is the second IDE controller, first one is JMicron I think).

My motherboard is a Gigabyte P35C-DS3R with F12 bios. AHCI is enabled and native mode (ports 0-3) is enabled.

Thanks for any pointers. This is the last thing I need to have a perfectly working Snow Leopard (and then wait for all the software to be compatible).