Sprint recently lauched a 3G/4G router named as Overdrive. It’s a WiMAX modem that connects to 4G broadband wherever the service is offered. Overdrive also has a fail safe mechanism to connect to a 3G network when the device isn’t in a WiMAX market.
Similar device would be suitable for LTE market as well. I haven't come across any such device supporting LTE/3G and acting as WiFi modem. Brining Overdrive like device to LTE will boost LTE growth and would offer subscribers uninterupted service in initial deployments. I would prefer support of HSPA+ as well in such a device.
Sprint also has also started offering an interesting bundle package in their accessories store. The 4G Case is designed to hold iPad, plus the the Overdrive.
With the Sprint Overdrive(TM) 3G/4G Mobile Hotspot, you can surf the web on your device up to 10 times faster than 3G. Sprint 4G covers 28 major markets such as Atlanta, Baltimore, Chicago, Dallas/Ft. Worth, Las Vegas, Philadelphia, Portland, Ore., San Antonio and Seattle. In 2010, Sprint will launch many more markets, and is expected to cover up to 120 million people with Sprint 4G service.