I have a friend having the same issue today. He said the phone seems to be working (rings, etc) while stuck on the screen, but of course he can't answer. BTW, he did install 2.2 this weekend, using the update.zip method. So... He plugged it in to his computer while it was stuck on the M screen and it "woke it up". Brought it out of the stuck screen and it has been working fine since. He just told me about this problem today, and just tried the USB plug in today so I don't know yet if it will be recurring. I used the same "update.zip" file to install 2.2 on my phone, 2 other guys' phones and his phone. His being the last one, all using my SD card installed on their phones. He is the only one to have this issue.
So... Not sure if this helps, but you're not the only one apparently.
