Having the same problem here using iTunes to burn an MP3 disc (not an Audio CD).
My CPU was pegged at 100% while burning a 8x (and I have a 32x burner). Given that burning a CD-R is an I/O bound task, there is no reason at all that the CPU should be pegged. Very little CPU is required for this operation.
So iTunes is most definitely hosed up. I burned 3 coasters in a row, and then decided to bump up the priority of iTunes. I think what's happening is that it's CPU hungry, but as soon as another application vies for the CPU, iTunes is getting choked because it wants it all.
After bumping up the priority of the iTunes process to "Above Normal", it can now get the CPU when it needs it and other "Normal" applications won't steal so much CPU. The downside of this is that your computer is essentially useless until the burn is completed, but at least I got a successful burn this time.
Would certainly like Apple to find a fix to this ASAP.