ffutures | Finnish Cellphone Providers?

I'm going to be in Finland for the Worldcon later this year, and I'd like to keep my cellphone working while I'm there. I don't actually expect to make any phone calls, but it would be very useful for things like maps. The snag is that I have a Freedompop account which is currently free, if I want to roam outside the UK it starts costing money and I've no idea how easy it is to get back to the free service.

Is there an easy way to buy a pre-paid SIM for Finland (e.g. at the airport when I arrive) that will be good for a week's stay?