Yes. My modem FAQ does explain how to connect a BlackBerry as a modem for a laptop -- however, to get the data to go through BES instead of the carrier TCP/IP.
However, if you must use MDS data instead of carrier TCP/IP, you need to use the third party Sharkmodem product at
www.mobishark.com rather than the modem built into BlackBerryOS 4.x on newer devices.