Rspec and BDD are about to get a lot more mainstream. Doing a little research I found out there are currently two Rspec books in development:

One is an Apress book called Foundations of Rspec: Behavior-driven development with Ruby and Rails by Chad Humphries and Ken Barker. Curiously, the FORBDDWRAR Apress book is only currently listed on the United Kingdom Amazon site. You won't find it on Amazon US.

The other book hasn't officially been announced yet but a simple google search found this where David Chelimsky, one of the creators of Rspec, will be putting out a book on the Pragmatic Bookshelf. It has not yet been posted as a Beta Book, but should be in shortly in the new year. Oddly enough, its titled "Behaviour Driven Development in Ruby with RSpec" :).

I can't wait for these books to come out. Rspec is going mainstream. Congrats to Rspec.

UPDATE:

Just watched The Rspec presentation and it turns out they announced the Pragmatic book at the end of their presentation. Q1 2008. I also recommend watching that presentation for some of the cool new rspec features including the new story runner.