Here's a link to an interesting post over at AnandTech on Broadcom demoing their new LTE chipset running in a Band 17 (700 MHz) and Band 4 (1700 MHz) LTE carrier aggregation mode (10+10 MHz). AT&T owns spectrum in both bands so it's clear whom this targets. Also have a closer look at the pictures at the bottom of the post as they contain some further interesting details.
And here's AnandTechs report about LTE carrier aggregation demonstrated by ST-Ericsson. Interesting in the pictures at the bottom of the post is the band combinations shown, lots for the US, one for Korea and one for Japan. A good indication of who's going to use it first and who's not on the list at all (Europe!).
While in Europe, many network operators have 20 MHz of continuous spectrum and thus don't need carrier aggregation some indeed only use a 10 MHz channel in the 800 MHz band and might be tempted to add another 10 MHz from some of their other spectrum holdings in the future. But I guess US carriers are more starved for it than their European counterparts, who are still doing fine serving their customers data hungry devices on UMTS.
Kudos to AnandTech for reporting from MWC off the beaten smartphone path.