It sounds like the dream of buying one LTE phone that will work with any carrier is not going to happen – at least not any time soon. There are several reasons for this.
Verizon stated to PC Magazine that their LTE phones will not work on other carriers’ LTE networks because the frequencies Verizon uses for LTE are different than the others. I find it hard to believe that this is an actual technical restriction. Verizon uses 746-787 MHz and AT&T uses 704-745 MHz which are right next to each other. It should be possible for the handset manufacturers (Samsung, LG, HTC, Apple, Nokia, and RIM) to build an LTE radio that would work for both frequencies ranges. There are multiband 3G phones today so this is not unheard of. Verizon is most likely requiring that phones they sell will only work on their bands meaning even if a multiband LTE phone was available, Verizon wouldn’t sell it to you.
Another factor here is the roaming agreements. Moving from one carrier to another requires agreements between carriers to work out technical and billing issues. I don’t know of any roaming agreements that have been established and it probably will be a while before any get worked out. This is more likely the reason that roaming won’t happen than technical issues.
The third factor to consider is the issue with LTE phones is voice. Neither Verizon nor AT&T are using LTE for voice, just data. Verizon uses CDMA for voice and AT&T uses GSM. That means that there will have to be separate devices for the separate networks unless they include three radios, which is unlikely. I expect that carriers want to carry voice over LTE at some point. Voice over LTE would let them retire the old CDMA/GSM networks. Also, LTE only phones could be smaller, less expensive, and have better battery life.
Personally, I’d like an LTE data-only phone where I didn’t have to pay for a voice plan. Skype or Google Voice can be used over LTE today to provide voice and texting capabilities. Any extra data charges would be erased by not having to pay for a voice plan. Sadly, I doubt that any carrier will offer such a plan. At that point the carrier truly becomes just a data pipe, and this is something they are desperately trying to avoid. There’s big money in those texting charges!
In conclusion, it appears that, as with 3G, your phone will be tied to your LTE carrier for the foreseeable future.