AT&T is clearly worried that T-Mobile's 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 they've subscribed to a smartphone plan, plus up to $250 more if they trade in an existing phone. There's no mention of when the promo will end, although it's described as a "limited time" offer. The deal may be tempting if you're already inclined to jump ship, although you'll want to do the math before you leave. While AT&T will now cut your bill when off-contract, T-Mobile's contract-free rates could be lower depending on the device you buy; it may be cheaper in the long run.