Some tidbits...

Last year, Bike Friday announced that they had sold their 10,000th bike after 10 years in business.
Dahon announced that their sales in 2004 were 230,000 units.
Dahon manufactures the the Trek and Specialized folding bicycles.

I agree that in middle America, folders have a ways to go. But in urban areas eff-J may be right. I have a friend with a Dahon Helios (http://www.dahon.com/heliosp8.htm) in NY and she says that folders are all the rage. She says she is constantly being stopped on the street by people asking about her bike. She lent it to a friend in Boston and the same thing happened.

Imagine being considered "cool" for riding a folding bike.