As a personal bookmark: There’s a pretty reasonable thread over on the Ruby on Rails: Talk list about BDD and RSpec. Reasonable arguments on both side of the RSpec fence:

Then I attended a lecture by Aslak Hellesøy. I mean, with so many people around raving about RSpec, I must be missing the point right? But I was let down: Alsak demonstrated it to be a wrapper DSL around Test::Unit for "people who find it hard to come to gripes with TDD". So it's a sort of workaround? I was disappointed by that statement of his. He says Test::Unit is tightly coupled to the actual code and that that's a bad thing. I say I like that coupling, because it keeps a sharp edge on the test-before-you-write principle: if you're mucking around code or tests that worked before, you'll find yourself pressed to advocate that change and express it in an additional or revised test.

then:

BDD comes from TDD, which comes from XP, which includes Refactoring as an integral part of the process. Refactoring is more expensive as your tests are more tightly coupled to the code, so one goal of TDD is to decouple the tests from the detail of the code (which is what changes most often). One thing that aids in this decoupling is focusing on behaviour instead of internal implementation. BDD champions this goal by putting it front and center. RSpec supports this by trying to use words like "describe" instead of "TestCase" and "should" instead of "assert."

and further RSpec support:

It's a small tweak, to refocus TDD practitioners a slight bit. They think it produces better specs/tests, not because it's something that couldn't be done before, but because it's now more obvious what you should be doing.