For some reason, USB modem support seems to have broken in 10.04. What worked effortlessly in version 9 now results in perplexing and frustrating non-detection of the device.

It mounts on the desktop as a drive, but refuses to register as a modem.

After two hours of trying everything I could Google up, including using gnome-ppp and making all sorts of amendments to config files and whatnot, usb-modeswitch turned out to be the answer.

Essentially, it stops the dongle from acting like a USB storage stick and kicks it in its modem hind.

Step 1: Fire up Synaptic or a terminal window.
Step 2: Download usb-modeswitch (you may have to get usb-modeswitch-data as well, so check that you did). In the terminal window, simply type sudo apt-get install usb-modeswitch.

That's it! Go forth and be surfy.