Surprisingly, a few years ago (2006), I received some mail from a PR firm who actually represented 37S. Whether or not they still use a firm, I'm not sure, but it's interesting to note that this success wasn't entirely built from their blog.
another reference: http://37signals.com/svn/posts/307-what-do-you-want-to-know
How do I remember all this junk and not stuff that really matters?!