FWIW, I pay 7.50 GBP per month prepay for the SIM in my Nexus One; that's for O2's web "bolt-on". Since I work from home, most of the time I'm actively using the thing it's on wifi, and while the background downloading it does (email, twitter, etc.) is over the mobile network, it's in no danger of hitting any limits. And it still all works out fine when I'm out and about.
Roaming charges here in Europe are a bit of a joke, however. I have an Irish Vodafone SIM for that purpose, which is 0.99 EUR for up to 50MB, per day. The best thing about that SIM is I've found it's still 0.99 EUR no matter whether I'm in the UK, the US, or anywhere in Europe (I even picked up Vodafone Albania while on a ferry between Greece and Italy).
So in practice, I find pre-pay SIMs combined with an unlocked smartphone works out pretty well.
Roaming charges here in Europe are a bit of a joke, however. I have an Irish Vodafone SIM for that purpose, which is 0.99 EUR for up to 50MB, per day. The best thing about that SIM is I've found it's still 0.99 EUR no matter whether I'm in the UK, the US, or anywhere in Europe (I even picked up Vodafone Albania while on a ferry between Greece and Italy).
So in practice, I find pre-pay SIMs combined with an unlocked smartphone works out pretty well.