Hi James,
I've heard other people mention that using the latest iPod Updater Utility to completely reset and also reformat the iPod nano may have some beneficial effect. You might want to try that before calling Apple Care (because they might not be too much help re: nano, it seems, lately).
That being said, however, I have had the same issues with my iPod U2 20GB for the last four or five months: Random sleep-coma after either me actively turning it off, or it going to sleep, regardless of Hold switch position. I recently tried the reformat/reinit thing, and it worked for a couple of weeks, but just today, it went to sleep and didn't wake up again without a reset. So it's a bummer. Every time I'm just about to think about sending it back to Apple under warranty, it seems to behave again for several weeks.
At first, I thought it might be an issue with my headphones (standard iPod ones) that may have a short or bad plug, etc; but it does its coma-sleep thing regardless of whether or not I have the headphones plugged in. So I guess not.
So try the reformat-iPod updater thing and keep us all posted. I'll also repost if I find the answer to my own issue.