What rom was that ? It's rather strange he got it at first boot as most report it was sudden and the phone was working correctly for a period of time.
Also i'm questioning, what about external sd's ? After reading the OP again i think we should also include if a external sd was present at the moment people got the error message and not just the rom. I'm no divine but i suspect there could be more involve then we think regarding the MTP, the rom used and external sd.
I know I am using a Vibrant, but I did not have an SD card in my phone when this issue hit me. It was sudden. I have been running ICS since it first was a ROM for the Vibrant.