All carriers are going to charge you extra for a data plan if you buy a smart phone. All of them. And they do this because, if you buy the phone from them as a discount or subsidy, they have to make the money back on the phone.
Think of it this way: why else would you want a phone like the MyTouch (or any Internet-capable phone, for that matter)? BECAUSE YOU CONNECT TO THE INTERNET WITH IT!
Most of the functionality on Android phones, Blackberries and iPhones requires a data plan. No, not *just* wi-fi, because wi-fi isn't always available. You won't be able to use the GPS, stream music, send email, browse the web, pretty much nothing, without a data plan. If you only want to make called and send texts, get a cheaper phone that does those things and doesn't require data.
Seriously.