Since I've upgraded my 15 gig to 2.1, the drive never spins down. I noticed that the battery was draining very quickly, and figured out that the drive was spinning. It doesn't matter if a song is playing or not, and the HD spins even when the iPod is just charging.
It also has a problem where it frequently reboots now. It does this on mp3s and AAC files that never had a problem before.