The css selector stuff is done by injecting jQuery into the page that’s being tested (if it’s not already there). This is pretty cool because it means you don’t need to worry about browser specific ways to select stuff (useful, since watir and selenium are/are going to be sat on top of webdriver so browser agnosticism is important). It also means that you don’t need to know xpath – cool if you’re a webdev since you don’t want to learn something new, and you probably already know css!
Check out the rQuery wiki for how to get started.
Overall, RQuery looks like a cool project. I’ll be following it for sure…