On Sunday (March 4th 2007), I signed up for a mobile broadband account with "3", one of Australia's tier-1 cellular UMTS/HSDPA carriers. It was a somewhat impulsive decision - the trigger being a significant drop in prices since I last looked, which was about a year ago.
For the Merlin XU870 to make a packet-switched data connection (ie. broadband), I needed to assign the username, password, and APN (Access Point Name). For the "3" network, the APN is "3netaccess", and the username and password are both blank.
Fire up minicom, point it to /dev/ttyUSB0, and the command is