Do I need to purchase a wind mobile phone if I already have a compatible phone on...

...a new activation? Planning to join Wind Mobile, I don't really like the phones they are carrying right now. Other then the Blackberry. I would like to get the Nokia N900 (which does work with wind mobile). I wanted to know if I still have to purchase a phone from wind mobile and then swap the sim, or can I just buy the SIM card and activate it on the N900. Thanks.
(I did contact wind about this never got a reply back)