Thanks for the suggestions! Before I read your post, I solved the problem (sort of)...
After some time on the phone with Plextor, I figured out what the problem was: CD-TEXT.
At firmware 1.06, I could write CD-TEXT without problems, using Nero 126.96.36.199. After the upgrade to 1.10, if I checked the box in Nero that told it to burn CD-TEXT to my audio compilation, Nero would hang, and the compilation would never write (or even simulate) successfully. If I unchecked the CD-TEXT box and tried again with the exact same compilation, all was well.
I downgraded to 1.09 but had the same problem. Reverting to 1.06 solved the problem; CD-TEXT again writes fine.
So, my question is - does anyone else out there even use Nero to burn CD-TEXT? Plextor claimed to have never heard of this issue (although they suggested I send a follow-up email with details so their engineers could investigate).