Well, I couldn't get my O2 Dongle to work (Huawei 1750 as recommended!!!), but I managed to get my Samsung Omnia 7 (Win7 phone) into USB tethered mode and connected up over my T-Mobile Web&Walk plan.
This may also work for anyone with a 3G service plan/phone if the phone has a tethering option (this means the phone can be used as an external 'modem').
For those with the Samsung Omnia 7:
Dial ##634# which puts you into Diagnostics Mode.
Then type *#7284# which will give you three options. Choose the second option: 'USB, Tethered call'. The phone will reboot.
(If you want to sync with Zune again you'll have to follow the steps above, but choose Option1.)
On the WITS A81G, go into the 3G settings (under Wireless & Networks) and you should see three options.
Long press on wcdma and you'll find all the options filled in. You need to change the APN which is called card (change to general.t-mobile.uk for T-Mobile), and the password (change to user for T-Mobile). LEAVE ALL THE OTHER OPTIONS ALONE.
Connect your phone to the regular USB port on the WITS and voila! Surf the web, use the market, it's all there.
Remember your data plan charges.....