I've been having this problem lately when my Treo 650 receives an SMS - it reboots. I thought the problem was specific to MMS, but seems like it could be either SMS or MMS. Here's how to fix it:

1) Use FileZ Treo application to browse Internal files.
2) Locate and delete "Messages Database". Be advised this deletes all SMS/MMS messages on the device.

That's it - oh, and a side note of interest: if you ever want to see the last error on the Treo, dial #*377 or #*ERR and you'll see the message. Mine was complaining about a UniqueID being duplicate.