I suggest rebooting after clearing and stopping service.
I have two xoomys and after attempting this at least 10 times, I decided to reboot after completing the second step.
After unlocking I went straight to update now and there it was waiting for me!
I started the download on that one and picked up my wifeys xoom.
On wifes xoom I cleared and stopped service.
checked for update, not there
cleared and stopped service
went back to check and there it was waiting!
I have a LG Lucid, and am trying this trick. I've tried about 15 times so far... I'll keep trying for a little longer. If it keeps saying no, I'll try tomorrow.
EDIT : It got to a point where it said "The software update encountered an error. Please go to settings and try again later" I guess that means that it actually got the update, but didn't update. And whenever I do that again, it's always errors. Help??