I think if you are on post-paid at&t, they will definitely add a data plan if you put the sim card in a smartphone. MVNO's won't do that, you will basically have access to whatever services you have prepaid for based on what plans they offer. So if you have data and the phone settings are correct, you will have access and be charged accordingly.
I don't know about t-mobile post-paid plans, though. With all of the changes they keep making I don't know how they would handle that but I'd assume you would have to have a data plan to be on t-mobile post-paid with a smartphone. However if you are on pre-paid they have no leg to stand on regarding trying to charge you more than what you specifically pick as a plan for your phone. If you only want pay-as-you-go, thats all you're going to get. Not sure how they bill for data on t-mobile pay-go, I don't think data is available with that.
Best bet is to stay with a gsm mvno ala platinumtel, airvoice, red pocket, h20, etc.
The whole bit about wi-fi not working is just wrong as stated, at least on iphone and androids. Not too sure about nokia, blackberries, etc. I remember having a nokia smartphone awhile back where the wi-fi didn't seem to work without having service, IIRC.