I just wanted to provide an update. We have two ESXi hosts running Adaptec 2405 cards and saw this issue on both of them. On Wednesday of last week I updated the one to the latest firmware, and the other was found to already be running the latest firmware. Since the update, they were running fine until yesterday at 1PM. We had a Veeam job replicating guests from one to the other, and both hosts shut down within minutes of each other. One of the systems restarted, and the last messages in vmkernel.log were of the same type that 92656_VMposted above. The other machine is still down and will be booted later today.
I don't really have any specific proof that it's related to the controller cards/drivers, but it certainly seems to be that way at the moment. They had been running ESXi 4 stably for ~2 years prior to the upgrade to 5.1, so the cards are probably fine...