I'm mostly just hoping the AT&T part is wrong heh
I'm not a gambling man and even I would put down money that if indeed the Nexus 3 is going to have an LTE radio the device will be coming out on Verizon.
-VZW and Sprint had their own answers to the N1, both of which were also made by HTC, but VZW is the only US carrier that lacks an answer to the Nexus S. There have been rumors of a white Nexus S for AT&T and of course you can unlock the T-Mobile one, but VZW has no option. I bet Google is going to make a preemptive strike to ensure VZW has an unlocked bootloader-possessing Google-branded device going into 2012 before anyone else can fill that option.
-VZW's LTE has been out for a while and has expanded in a multitude of areas and AT&T's is going to be brand new. What the heck is the point of a Nexus 3 4G LTE if you can only use the 4G bandwidth in less than 5% of America? It doesn't matter if T-Mobile gets bought - AT&T still has to build out the network and that will take another year or two to catch up with modern-day VZW's network assuming the former grabs T-Mobile's spectrum.
-VZW has plans to get rid of their 3G network by 2015 or so and even stop selling devices with 3G radios in them as early as 2013 (they plan to have 2/3 of America covered with LTE by then). People who buy a Nexus at the end of this year will be stuck in a 2Y contract, which will expire at the end of 2013. Not only does that imply that the LTE radio spec is probably legit but also does it line up very nicely with the Nexus 3 being on VZW's network over or in conjunction with that of AT&T's.
Google hasn't stopped pushing the idea of a carrier-neutral device. They will get to Verizon, and LTE may just be their ticket in. Also, VZW reps do read the interwebs. They know how badly many VZW customers want Nexus devices. They may be old school Ma Bell types, but they are also very well aware of the fact that their devices have always been subpar. For every DROID or Android device, another carrier has released a significantly better competitor on their networks (only real equality was DInc vs N1, only real victory for VZW was DROID 1). This time they may be able to at least be on par with everyone else with a Nexus device.