Hey, you're right. I kept it unplugged for ~5 minutes and it spun down.
"Playing DVDs on a portable computer
If your portable computer has a DVD drive or is connected to an external DVD drive, you can use DVD Player to play DVDs on your portable computer. To preserve battery charge when you’re not connected to a power supply, DVD Player loads a portion of the movie into your computer’s cache to play the movie and then puts the DVD drive to sleep. When DVD Player needs to load more of the movie into the cache, it wakes the drive. Typically you are unaware of this behavior. But if you scan ahead to a portion of the movie that is not yet cached in memory, it might take several seconds for the DVD drive to wake.
The DVD Player preference that puts the DVD drive to sleep when a movie is playing is set by default. To turn it off, choose DVD Player > Preferences, click Player, and then deselect the “Put the DVD drive to sleep” checkbox."
It would be cool if it would cache even when plugged in so the disc will spin down..