I was browsing through this excellent presentation given by Dave Astels and David Chelimsky at RubyConf2007. Turns out that David Chelimsky and Aslak Hellesøy are working on a book about Behaviour Driven Development using RSpec.
The presentation is an excellent introduction for anyone thinking about moving to RSpec. It tells you a bit about the (rather short) history of the framework and the differences between BDD and TDD.
It also shows off the awesome plain-text stories I posted about earlier.