I must have been living in a cave or something, apparently, there is a famous hack for Tmobile customers to sign up for a $5.99 unlimited data WAP/T-Zone plan and turn it into the full-on internet package ($19.99 value) allowing full HTTP/HTTPS and IM access.
Simply Google the phrase "Tmobile $5.99 Tzone Hack", you'll find plenty of references. Apparently, folks with various HTC smartphones (ie. Tmobile Wing, Tmobile Dash) and jailbroken iPhones have been taking advantage of this. Hackszine has a coverage of this and a couple of links for the instruction for iPhone.
I have always believed the current cost of mobile data is too expensive and one day, unlimited mobile data plan will be in the range of $5-10. Some carriers are currently charging upwards of $35-40 which is ridiculous. Apple iPhone Data Plan ($19.99), AT&T MediaMax 200 ($19.99) and T-Mobile's unlimited internet plan ($19.99) are leading the way to lower monthly charges, but I see some ways to go before mass-public can afford to be more engaged in the mobile convergence. This is the second time I've noticed about Tmobile's advance thinking on data charge strategy, I've written about their $9.99 unlimited email plans once.