And that could be vice versa too, with Verizon Wireless stating that their phones won't "be compatible on other LTE networks in the US," owing to their "different frequencies." That may sound strange as both Verizon and AT&T use the 700MHz band, but there's little cross-over apparently, with Verizon using the 746-787MHz area, and AT&T on 704-746MHz. I don't care how they do it, but they're just going to have to fix that incompatibility then, won't they? [PC Mag]
You really wouldn't want to transfer back and forth with Verizon and ATT LTE anyways, as Verizon's falls back on their CDMA network if in an area that doesn't have LTE, and ATT's falls back to UMTS/GSM. An ATT/VZ phone would need like 6 or more total radios in it.