I posted this in another thread and am copying it here since it is relevant:

Just got my i760 yesterday and this is also a concern of mine. I do pic/vid messaging and noticed that when you receive an mms message it would stay connected/dormant. This connection stays connected for 24 hours before it is dropped. I know that no data is sent back and forth while dormant unless I open a browser and start surfing or whatever. My problem was if I had gotten an mms message I would be connected to bba and not realized it and then opened a browser, it would default to the dormant bba connection instead of wifi even if wifi was connected. So I called tech support and got a tech that knew exactly what I was talking about. He changed the 24 hour timeout to drop the bba connection to 5 minutes which is the lowest it can be set. I tested by sending myself an mms and waited. I tapped the connection in the top bar where it tells you how long you have been connected and waited for the disconnect. It dropped itself at about 6 minutes. This is better than nothing and accidentally using the bba when I wanted to use wifi. Another new thing I didn't notice until after he made this network change is when I tap the connection in the top bar, I now have a disconnect option along with hide which I am 99% positive wasn't there before. Someone correct me if I am wrong there though. I had been going in to the wireless manager and turning the phone radio off and back on to disconnect the bba.