According to this:

http://www.marketingvox.com/55-of-us-adults-have-home-broadband-but-10-use-only-dialup-039691/

in April '08 dialup in the USA was at ~10%. Given the preceding two years from '06-'08 saw a dialup drop from ~25% to 10%, it's probably a a decent bet that dialup is now somewhere in the ~7-5% range, possibly slightly higher due to the recession.