FYI: ref MMS on Boost: always been wierd, always needs a fix. Found one that works for me on Nega WP7 3.5
http://forum.ppcgeeks.com/1864910-post38.html
I can attach something to a txt msg and send it to my phone. I think there is a small size limit (may be 5K). Inserting a pic into a txt message does not work(probably not supposed to).