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.
Actually this seems to be using the mobile email (or other email) to send the messages with attachments. The only useful thing I can see is the option to show delivery confirmatio as in PIN messaging.
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).