I don't know if this is a HDD problem or what, but it has happened to me multiple times with the 4gen ipods.
Previously, a 4th gen 40gig iPod I bought from a headfier would just freeze while playing. It did this intermittantly despite tips from the Apple folks. I used the updater (formatting it, reinstalling the newest firmware), I tried hard-booting it, yadda yadda. I ended up sending in that unit twice. On the 2nd or 3rd service call they replaced it with a (new?) unit.
This new unit has seemed to perform fine for the past 4-6 months. However, yesterday, it did the exact same thing. The iPod just froze during one song. I can't reboot it. Plugging it into my PC to recharge it does nothing (it won't recharge and is not recognized by my PC). I just set up a service request through Apple.
While my old, 2nd gen had a few probs to begin with, it ended up fine. This 4th generation, IME, has just been problematic as hell. I love my iPod, but am getting severely pissed.
(please, no comments about buying a different DAP instead)