OK, I've had the infamous App Error 523 SIX times in 13 months. After being elevated by Tech Support from T-mob over to Level 2 support for RIM and that tech consulting level 3...here's the cause:
You have to have all 3 conditions on your phone:
1) Using the Today Theme
2) Content Protection Enabled
3) Phone is in locked mode
Then if you receive an MMS...wham!
They know about it & are trying to fix it. My solution is to just turn off the Content Protection.