I have an O2 card at £14.50 PCM, not unlimited but I have never had to pay extra in over a years usage (that includes 2 weeks on board during which time I did a lot of work) and niether has my collegue with the same set up and he really hammers it, BTW you can often find WIFI spots for free also.
Also with O2 you can get an extra free number attached to the same SIM card to use your Laptop as a fax, I never use it for outgoing but it is really handy for incoming.
BTW you can leave the pcmcia card connected as you do not pay for "air time" but only for transmitted data.
As a related aside I have found BT Openzone the best value for WIFI hot spot usage.
Still paid no extra on our O2 Cards for fairly heavy use in : China mainland, Antigua, France, Germany, Switzerland, H.K. Did notice that when I put the SIM ito a phone then was charged roaming.
Don't know.... but never had a problem finding a willing 'unsecured' connection upon the average high street. (Yeah I know boats don't go down the high street) but....it's a friendly idea really, sharing your broadband to the world passing you by, and some don't even know they are doing it. But now I just want to borrow a decent car, a flash boat and a luxury house in the med would be nice too. Pity the doors aren't let wide open on these aswell!!!
Mike