Since we've been supporting 4G modems in our ACCESS and LiveShot products, we've been on a search for a USB modem that works worldwide on any network. The problem is that, while LTE is a good standard, every region has different LTE bands active and no USB modems exist that cover everywhere. The worst offender is the US, where each carrier occupies a hodge-podge of different bands, not to mention the different 3G standards deployed. So there isn't even a common American modem out there (it would certainly make development easier if there were!)
But there is one device that contains an unlocked LTE modem compatible with virtually every 4G network worldwide--The Apple iPad Air. Apple made the decision to make only one version of the Air for use worldwide, and the compatible band list is mind-boggling, covering every 4G and 3G network we've ever encountered, including every American band and modulation. And it's only sold unlocked, to be used worldwide.
It's not as elegant as a USB modem, but the Air can easily be put into "tether" mode, allowing Comrex gear to connect to it via Wi-fi, and sharing the LTE connection.
I've done my share of Apple whining here, but this is one time they did us a solid.
Edit 9/10/14
Neglected to mention that the latest iPad mini also features this latest universal LTE modem, as well as the newly announced iPhone 6. Now we just need one in a USB form factor!