I think that 37signals has been in business since the late 90s, so I'm not sure I'd qualify a business that's been in business for 13+ years as a startup.

I like Steve Blank's definition of a Startup -- it's an enterprise set up to search for a profitable business model. 37signals is long past that.

It was never that to begin with. Fried wasn't trying to create an enterprise. He just wanted to own a small business. That's what he has.

An enterprise is not necessarily enterprisey.

Some enterprises are fictional starships.