As I write this, I'm on hold with Dell tech support.
It seems that my blade enclosure's DRAC card has somehow become detached from the Avocent KVM card. The general configuration screen shows the card, but none of the management interfaces are able to reach it.
Apparently there's a reconfiguration necessary through OpenManage, which I haven't set up yet, so we're installing it now on my XP VM. Hopefully this will reconnect the two feuding devices.
2 hours later, the problem has been resolved. I had to issue racadm commands from the serial console.
the lesson here is: If you're going to change the subnet your blade enclosure is on, change the KVM module's IP first, otherwise everything goes to hell.