I know Sprint is the only carrier where we have (to my knowledge) information about which vendor is handling what part of their network, but I'm curious as to whether Verizon, AT&T and T-Mobile have similarly regionalized their next-gen network deployments.
For reference, here's who's building out networks for the Big Four, plus C-Spire and MetroPCS (since we now who they're using to roll out LTE):
Verizon - Ericsson, AlcaLu, NSN
AT&T - Ericsson, AlcaLu
Sprint - Ericsson, AlcaLu, Samsung
T-Mobile - Ericsson, NSN
MetroPCS - Ericsson, Samsung
C-Spire - AlcaLu
AlcaLu = Alcatel-Lucent
NSN = Nokia Siemens Networks
...and yes, Ericsson is really busy rolling out LTE, it seems.