Just get a 3G phone with bluetooth, and whatever service the network offers. I'm with Three in the UK, and use the 1Gb mobile broadband addon for my phone, and it works very very well I managed to get a free 3G usb modem, but have not managed to get it working, and as another poster has said it will need external power from a powered hub.

With Three you can go PAYG as well for the same deal (1GB/tenner) b ut it expires after 30 days. With Three on PAYG you actually get 150MB internet allowance, which may be enough for the odd update to twitter and last.fm etc.

Other networks such as Virgin, T-mobile do PAYG on a daily rate of maybe 50pence/day.