There's a common misconception out there that I'm part of the YUI team. Well, I'm not (I work on My Yahoo!), but that didn't stop me from submitting some code to be included in the 2.3.0 release (details). I've been secretly working on this little creation for the past few months and now that the release is here, I can finally talk about it.
I'll be talking more about YUI Test in the coming weeks and months, but for now, check out the documentation and I hope you enjoy the wonderful world of unit testing.