Today when it happened (at least the fourth time) I called AppleCare to see if they had any details on it. We found a Safari plugin (I don't even use Safari!) from Leopard or Snow Leopard to be the most likely culprit!
In my user account login items, SIMBL Agent was listed. We deleted it from that list (it's apparently known to have issues causing Lion to freeze up), but found that it still ran in the background when I restarted. Se we went hunting and found it on my hard drive under Library\LaunchAgents. Deleting it there stopped it from starting up on a restart.