Just got back from the datacenter. Everything was running again about 60-90 minutes ago.
Since the last reboot the server in question had more disks installed. The disks are split across two 3ware raid controllers (one with the new disks and one with the old). The bios decided to try booting off the controller with the new disks – of course there's no boot record on those so it just stalled.
The serial console was working, but for some reason it'd not show me anything until it had skipped past the "press button to enter bios setup" window. Robert could get into the bios remotely, but he was too lazy to check the settings so downtown I went. Grrh.