My Kindle 3 just crashed twice in a span of about five minutes. Wireless (3G) was on at the time, which I leave off most of the time. I was scrolling through the [Menu] menu both times, I believe.
The first time, it restarted on its own. The second time, it seemed to be hung up and eventually it responded to my holding the power/wake switch closed and it restarted.
I googled “kindle 3 crash” without the quotes and the first hit is this report, which seems like exactly what I encountered.
12 December 2010 Update: No recent crashes noticed whatsoever.