Well, I downloaded this and installed it into my BB without a problem. The device rebooted. A new menu item "SMS and MMS Messages" appeared. and an Xpress Messaging icon. I sent a picture attached via MMS to my girl's BB 7290 (with Xpress installed) and she got it no problem. I will be playing with this more today. My only concern with Cingular is the incursion of additional fees.
Xpress Messaging has nothing to do with MMS. It is a seperate application that requires an add-on feature with a monthly fee. It gives delivery and read receipts to other XM or Interactive Messaging Plus users (aka RIM 950/957 w/Cingular now Velocita Wireless).