Repair permissions, then perform an SMC reset:
Resetting the SMC (System Management Controller) on Intel-based Macs:
Resetting your Mac's PRAM and NVRAM:
I'm having the same issues after a Snow Leopard installation on a 2006 MacBook Pro.
I repaired permissions, which were not all correct, then restarted, and DVD was stil not recognized.
Then I reset the SMC, restarted and DVD was still not recognized.
I was able to get a DVD inserted before another cold start, and it spooled up and appeared after system startup. Then I removed that DVD and inserted another and it was not recognized.
So it does appear to me that the DVD drive is working fine, but I am still not able to find the right fix. At least there appears to be a workaround, which is to get the DVD reading at or during boot.
I haven't yet taken the step to reset NVRAM.