Qualcomm chip that we had heard about which would have all the 4G radios on one chip. According to Apple, the iPhone 5 will support HSPA+ (it's unclear if that includes T-Mobile bands), DC-HSDPA, and LTE. So, as Apple says, it's one chip, one radio, and a dynamic antenna to switch between them to get you the best signal.
Apple then went on to list the carriers supported, and it does cover quite a large number of carriers across the globe. Apple specifically mentioned Verizon, AT&T, and Sprint in the US; Telus, Fido, Rogers, Bell, Virgin, and Koodo in Canada; KDDI and Softbank in Japan; KT and SK in Korea; Smartfone in China; and T-Mobile Europe, just to name a few.