Of course we should take these unofficial dates with a grain of salt. There's always the possibility that, if these are the official dates, they could easily be delayed.
While this is great news for early adopters in those markets, we don't know what the rest of their launch timetable will look like. And 4G devices will slowly trickle to market. Much like with Verizon's LTE, our first tastes will come in the form of USB modems and mobile hotspots.
If the T-Mobile/AT&T merger is approved, then the resulting AT&T giant will be the carrier to watch in 2012. Even without exclusivity on the iPhone or iPad, their LTE service and likely swath of new devices will make them very exciting indeed...And that's exactly why regulators (and Sprint) are concerned.