ATT is clearly worried that T-Mobiles recent success may be coming at its own expense — it just launched a promotion that gives T-Mobile customers up to $450 to switch to Big Blue. The deal offers converts a minimum of $200 once theyve subscribed to a smartphone plan, plus up to $250 more if they trade in an existing phone. Theres no mention of when the promo will end, although its described as a limited time offer.