The Verizon / AOL deal has brought up an interesting data point: AOL still has 2.1 million dial customers ( page 4 in the PDF ).
Combined this with the number of Internet users in the United States ( ~279 million ) you get: ~1% of United States Internet users are on AOL dial up.
I ran www.google.com through WebPageTest, comparing Cable ( 5 Mbps down, 1Mbps up, 28ms RTT ) to 56k dialup ( 49Kbps down, 30Kbps up, 120ms RTT ). The result:
And google.com is fast compared to most sites, nearly every modern web site is going to be horribly painful on a dialup connection.