How many ISPs are there in the US?

Answers vary but this source says 3500 to 4000 ISPs

Amazon.com/ISPs-United-States/AnswerViewer.do?requestId=524267">http://askville.Amazon.com/ISPs-United-States/AnswerViewer.do?requestId=524267

and this one says at least 7000

http://pcworld.about.com/magazine/2306p080id120341.htm