Last night when I got the 2.2 update on my X, I got stuck with the M during reboot. I took the battery out for a while as I cleared my SD Card of the previous update.zip file, thinking that was the problem. Today durind the day I somehow got stuck with the M again without even rebooting the phone.
I just got off the phone with a tech at Verizon Wireless and he told me that this was an issue with this update and that I need to take the phone into a Verizon Wireless store so they can Flash 2.1 back to it (this version of 2.1 has "pieces" of 2.2 in it to prevent this issue) then pull the 2.2 back down. He said there is also a patch coming out in a week or two to fix this and other issues.