Typically these USB dongles are designed to appear as USB media devices (e.g. pseudo CD-ROM) and one has to mode switch it to a USB modem. Unfortunately I had a Huawei E1552/E1800 which required some USB mode switching magic, but to find this I first required internet connectivity. Fortunately I had a sacrificial laptop which I installed an old version of Windows XP which allowed me to then connect to the internet using the 3G USB dongle and I was able to then track down the appropriate runes. OK, I feel bad about installing Windows XP, but I was being pragmatic - I needed connectivity!
The procedure to get this device working on Ubuntu wasn't too bad. First I identified the USB dongle using lsusb to get the vendor and product IDs (12d1:1446):
Bus 002 Device 013: ID 12d1:1446 Huawei Technologies Co., Ltd. E1552/E1800 (HSPA modem)
Then I added the following runes to /etc/usb_modeswitch.conf -
..this appears in many forums on the internet, kudos to whoever figured this out.
Then I ran "sudo usb_modeswitch -c /etc/usb_modeswitch.conf" and this switched the dongle into:
Bus 002 Device 012: ID 12d1:14ac Huawei Technologies Co., Ltd.
..and I was then able to simply connect using network manager. Result!
** UPDATE **
Mathieu Trudel-Lapierre fixed this (9th Feb 2012) and now Ubuntu Precise works perfectly with the Huawei E1552/E1800. Thanks Mathieu!