Does anyone know of such a device?
It depends where in the USA you are going. AT&T has 2G/GPRS/EDGE and 3G/UMTS at 1900mhz. They also have some limited DC-HSPA deployment which they call 4G. (not to be confused with LTE/4G). T-mobile has 2G/GPRS/EDGE at 1900mhz, but is gradually converting some of this to 3G/UMTS and in some cases DC-HSPA in some cities/markets.
T-mobile also has the 2100/1700 split band, which is much harder to find a MiFi that will suport. This band is where they have 3G/UMTS and DC-HSPA 4G.
Three doesn't have an LTE/4G roaming agreement with anyone in the US yet, but might do eventually, but LTE bands are yet another problem.
Note that USA DC-HSPA speeds are nowhere near as high as in the UK with Three or EE. 10megabit/sec would be 'very fast' for them.
James BT Infinity 2
19/09/2012 - Sold 42/6, Now 52/9, Sync @ 55 / 9.5 Mbps @ 470m approx
19/5/2014 - just ordered PlusNet Unlimited Fibre - awaiting activation date
15 years broadband (1999 ntl:cablemodem trialist) - Router: Asus RT-AC68U (merlin) - Modem: HG612 unlocked Typical BT speedtest