Fair’s fair, so it’s only right that I should report the fix which resolved the problem for me. I can’t guarantee this will work with every netbook/notebook configuration (in my case, E620 dongle and IBM Thinkpad T60) but all indications are that it works for the majority, if not all cases.
So courtesy of dj_toons, here’s the fix.
Plug in your USB dongle, and boot your computer.
Open up Places|Computer. If you see a virtual CD ROM and an SSD card slot, right-click on the Virtual CD ROM (you don’t need it, the only reasons it’s there is to supply drivers to Windows users) and select Safety Remove Device.
Once the SSD card reader and virtual CD ROM icons have dissapered , close the browser box.
Open a command line terminal [Applications|Accessories|Terminal]
Copy and paste the following commands (you will be prompted for your password).
sudo rmmod usb-storage
sudo modprobe -r option
sudo modprobe -r usbserial
sudo modprobe usbserial vendor=0x12d1 product=0x1001
sudo pppd ttyUSB0
When you go to Network Manager, your modem should be listed.