If you are in the UK you may want to take a serious look at the new pricing for the mobile data dongles from Hutchison 3G (aka Three).
The price of the dongle has been halved and the plan prices are very tasty.
Now, what makes 3G data interesting to me is this 3G/4G data router that has Voice QoS built in. By combining a new HSUPA router, and unlimited data plan and a data card like the new Sierra Wireless HUSPA 880/881 wireless modems, you could just about have all the VoIP you need wherever you go.