Nokia Lumia 800 passes through FCC with AT&T 3G bands 850/1900 MHz bands but not T-Mobile AWS spectrum support. Probably AT&T would announce this device or its partners would pick-up to sell through their channels in a few weeks with a carrier subsidy. I anticipate AT&T would price this device for $199 with 2 year contract once they start selling the device. Engadget has more information about the tear down of the device at FCC Labs.
It is not clear whether there would be another Lumia 800 with LTE spectrum support as mentioned by CNET few days back. Also it is not clear that Lumia 800 with LTE would be called Lumia 900 or Lumia 900 is going to be a total different device. Anyway I hope we will know more about these during CES 2012 or MWC 2012. Stay tuned.
Sources: WMPowerUser, Engadget, FCC