I'm having a similar issue -- I have 3 Apple TV2's in various rooms of the house, and only one experiences this issue. It often happens 1-2 times per week while viewing Netflix or Streaming from iTunes -- the unit reboots itself and comes to the original pre-setup menu (3 menu options) and no stored network information (security key, network name, etc). It's annoying having to re-enter the security key so often.
Also, the unit makes a loud popping sound through the TV speakers or home theater speakers quite often while browsing through menus. The popping goes away once media is in play.