Well, they use wifi hotspots, and not mobile web, so I'd assume that if you bought the phone it'd be an option on the phone vs your network, so it should work on ToGo phones (which I have).
I doubt it'd be on the cheap phones they give you at a shop for prepaid and all that, but if they give away free razr's then i'd immagine that one of the options they'll have will be wifi phones.
I am looking to upgrade my phone, Nokia 6133 (well, I'm hoping for a 6131 for MP3 ringtone support which no one offers anymore due to DRM). I have a Nokia 6102 right now that I had to settle on.