I think it is a glitch in how the OS handles MMS.
I have had this issue. I know others that have had or have this issue. Each are from different messages.
Generally, removing the battery fixes the problem, it does not always work though!
An OS re-load does not always work either.
When I bought my Tour from Verizon, it did not have OS 5.0 on it. MMS messages I received with the OLDER OS didn't have this problem. However, once I upgraded to v 5.0 the same messages from the same senders will now cause my phone to have this problem.
Since this started happening, i have done an OS reload, and had this issue come back, with a different MMS than what it had happened with before.
So the "RELOAD OS" theory isn't exactly the fix, but congratulations on making the forum sound like a Microsoft Windows Help Desk