it was opening these mpeg4 files directly from e-mail (as long as they are under 10 megs) a couple of weeks ago.
That sounds like a winner! I bet it has something to do with the available memory. Have not seen any written stuff about this but I bet if you play around with the memory allocation i.e. remove some apps and get your available back up higher it will start playing them directly through the email. Thisis only a guess mind you and I have not personally experienced the issue myself. I may try and send myself a video and see what the behavior is.