T-Mobile to debut unlimited mobile-to-mobile plan next month?

Here's a juicy little rumor that's sure to excite the T-Mobile faithful. Retail training materials, acquired by TmoNews, indicate that the nation's fourth largest wireless provider may launch an "Unlimited Any Mobile" add-on early next month. The feature can be tacked on to existing T-Mo plans (with some exceptions) for a paltry $10 a month. Once added, you're free to call any US mobile without having to think about minutes, and there's no contractual commitments to the package. Sprint patrons have enjoyed this luxury for quite some time, and AT&T users can get the same, with the adoption of an unlimited text messaging plan. If this does materialize, maybe it'll help woo some of those customers back.