I have been waiting for Cradlepoint to come out with there external modem to be LTE compatible. There are now 2 devices that are capable. Here is some info from Verizon in regards to the problems with the handing off between 3G and 4G.
Verizon LTE Current Issues If you are using or considering using a Verizon LTE modem (Pantech UML290 or LG VL600) - especially if you want to use a router - read this page for important information on the network-wide problem Verizon is having and how it may affect you!
This page will continue to be updated as more information becomes available. Last update: 1/3/2011
The problem: Verizon's 3G/4G modems are having trouble with the "hand-off" between the 3G network and the 4G network. Simply put, users are finding that their modem works fine when locked to either 3G or 4G mode in a particular location, but when the modem is set to "auto" (i.e., when the modem is set to automatically connect to the fastest network available - 4G or 3G - this is the default setting on the modem), it has trouble making the switch between networks. Verizon has acknowledged that this is a problem and is working on a fix. For the time being, Verizon recommends changing your modem's settings from "global" (AKA "auto", where the modem will search for the best network to connect to) and instead lock it into LTE mode if you are in a 4G area or EVDO/3G mode if you are in a 3G-only area (click here for instructions).
While this problem can obviously affect any Verizon 3G/4G modem user, it's especially frustrating for UML290 users who want to use their modem in a router. Both the Nexaira BC2 and Cradlepoint routers running the beta LTE-supporting firmware support the UML290 in LTE mode, but if you are using the UML290 in 3G mode, you must lock the modem to 3G mode first before it will work in the router. This is due to the way Verizon's network handles the hand-off from 3G to 4G. The VL600 remains unsupported by any router in any capacity.