If the $50 unlimited rumor is true, the implication is that T-mobile is willing to cannibalize the revenue stream coming from it's high-value power users in order to attract more lower-value consumers. To close the thought, that would further imply that the data hungry "power user" really isn't a big part of the T-mobile customer base to begin with.

Edit to add: It seems risky to offer unlimited data as part of this package from a financial point of view, especially since separate data plans can potentially add incremental margin (especially with regards to subscribers with the premium phones).