is this Iphone driven ?
If AT&T originally had no plans to deploy LTE soon, and is now fast-tracking that project for 2011 deployment, my gut tells me it is because Verizon's LTE deployment will be a threath to AT&T.
Networks have to think longer term. They want to have the next generation network with credible coverage before they start to lose customers because the can't carry the latest and greatest handsets.
And if there is to be a great iPhone LTE party next year, AT&T doesn't want to be absent.
I suspect that LTE is an easier upgrade for networks already on a GSM stack.