Update: I take it back. Apparently I missed the part about the DOCTYPE needing to be set to XHTML. Once I did that, MooTools loaded like a champ. I'm rockin' the Fx and Transitions package now to mimic a slide in and fading background color effect after an AJAX request has been sent and the newly inserted content is posted back to the page ala GMail.
Apparently Doug1 thinks he sent me a link to MooTools. I don't recall that being the case. Either way it is a slammin', lightweight DHTML/AJAX library that I fully intend to spend more time looking at. I think I like its examples more than any other library I've seen and now that it's been blogged, I have a better chance of being able to follow up on that intention.