Actually it will take an update unlocked (at least 3.2.1) and it will relock it. My other Xoom just did it this morning. Stock unlocked, I'm not positive it relocked it but during the update it said "Securing bootloader - OK" or pass or something like that.
You shouldn't need too. And like mentioned it probably shouldn't be unlocked prior to the download. Although mine did work.
Have you guys tried a simulated battery pull/reboot (power + volume up with the Xoom on) then try it? Make sure you have a full charge too or plugged in.