I tried burning a playlist Id just made to a CD in iTunes 11 and got the error message:
“disc burner in use by another program”
Id only put the CD in and couldnt see any other apps running which might be using the burner, except maybe Finder was doing something with it?
If you get the same error, simply eject the blank disc, then tell iTunes to burn the playlist before inserting the CD again. It seems once iTunes is waiting for the disc then everything goes as planned.