According to an FCC report (PDF) on the state of Internet access in the United States, 68% of high-speed connections are too slow to be considered broadband. TechSpot has a summary of the report, which points out that an Internet connection must be capable of 4Mbps down and 1Mbps up to be considered broadband by the FCC. Surprisingly, 58% of of households are stuck with less than 3Mbps of downstream bandwidth. 30% do have a connection that offers more than 6Mbps down, though.
Obviously, the sheer geographical area that must be covered by US broadband providers has a hand in keeping speeds down, at least when compared to countries with higher population densities. However, the report covers the Internet connections that folks have chosen rather than simply what's available in their area. The FCC's definition of broadband may require 4Mbps down and 1Mbps up, but some are choosing to get by with less.
Interestingly, the number of high-speed Internet connections for residential homes went up by 3% in the second half of 2009. Over the same period, the number of wireless data plans increased by a whopping 53%. For consumers who don't ask much more of their Internet service than web browsing, email, Facebook, and the occasional YouTube clip, a 'net connection that falls short of the official threshold for broadband may still feel fast enough. Having that Internet connection available at all times via a smartphone is probably more valuable than a faster connection at home.