I was about a yard away from 2 different AT&T HSPA+ cell sites and the highest I got was 5Mbps. I believe time of day is going to have the most effect on speeds. AT&T has a lot of people using their network which results in slower speeds. Last night I ran several speedtests on my iPhone 4s but at 1 in the morning and was averaging 5.5Mbps when in the middle of the day I average 1-3Mbps.
How many bars did you have for your signal?