The Lumia 900 requires a microSIM and the only microSIM I have is in my main phone, the N9. Not that I get why it couldn't use a good-old miniSIM, the phone is Huge :)
When I had the Nokia 603 for testing it never occoured to me that I could have gotten a pre-paid service so I could have tested the phone features properly. You might wonder why, but I've never used pre-paid service, I've been using the same service provider since 1996 (my first mobile was a Nokia 2110) and usage of pre-paid is not that common here in Finland, afaik.
So, today I got a pre-paid service from Saunalahti, only 6,60€, a good choice if you ever need pre-paid service here in Finland and it even includes data service with a special price of only 0.66€/day on microSIM phones.