There are a few different variants of the Sony Xperia tipo. There is a model with dual SIM cards called the Sony Xperia tipo dual (which didn't visit the FCC). In the U.K., the Sony Xperia tipo will be available next month SIM-free for £134.98 ($211). The model seen at the FCC did have 1900 / 850 MHz WCDMA connectivity, making it compatible with AT&T's 3G coverage. While Sony hasn't yet made an announcement on whether this model will be released in the U.S., the model number of the phone that visited the FCC would seem to indicate that it will be sent to the States. The "a" in the model number ST21a usually indicates an American model for Sony.
source: FCC via UnwiredView