Windows Media Center cannot play this DVD. Please Restart Windows Media Center and insert the DVD again.

What a great error message.  I’m not sure what update I could have installed that would prevent me from playing DVDs in my Vista Media Center PC, but one of them removed that ability.  The fix(es)?  Re-enable UAC or map your DVD folders to a network drive.  Why this only affects local media (and not network media) is anyone’s guess.  I opted for re-enabling UAC as this seemed easier than remapping 4 external USB drives.