UMTS is ATT's basic 3G service...and is generally a very slow network but has a big coverage base....
HSPA is the newer 3G service which gets even better speeds but it still limited in some areas.....An iPhone as you mentioned will connect to UMTS or HSPA depending on cell tower location......HSPA+ which is what the INFUSE connects to is nothing more then HSPA with better backhaul for higher speeds but has the same coverage and range of regular HSPA.
HOWEVER, the way it works is that it connects to the STRONGEST signal at the time, if UMTS is outputting a stronger signal from a cell tower you will latch onto that one more often then HSPA....
At work and traveling to and from, i am usually on HSPA, but at home that signal is weak (location of cell tower with HSPA amplified) so i am usually on UMTS...
Samsung Galaxy S III
Samsung Galaxy S II
Samsung Infuse 4G
Samsung Galaxy S Captivate
Samsung Galaxy S Vibrant
Samsung Galaxy S I9000