I'm in a similar situation. I'm going to various contrinues in Europe for 2 weeks. I've been looking at sim cards, but most of them seem rip-offs, especially the one Chrissy posted - £10 for a sim card with no credit, so like £20-£30 minimum for 2 weeks, just not worth it.
This one
http://www.free2roamsim.com/ is £10 and you get £10 credit with it - the downside is that delivery is £5 for some reason...
So I'll keep looking for a better deal.
What cellphone/mobile do you have (Nokia/Samsung/Sony Ericsson/etc.)? Doesn't matter which
network provider you're with at the moment, if your mobile is unlocked (doesn't cost much) [edit]
and is dual-band compatible you can put any sim card in it, including these European throw-away-after sims.