With version 3.1 we will have querySelector and querySelectorAll but version 3.0 is still the most used one, FireFox speaking!
The nightly build of vice-versa introduces a new file, mainly used for personal experiments via vice-versa library and the first experiment is an improved document.query for those browsers with document.evaluate support without document.querySelectorAll.
Unfortunately Internet Explorer < 8 does not support XPath queries over (x)HTML but at least for FireFox 3.0 I have obtained best overall performances via SlickSpeed Selector Speed Test comparing latest version of each library such Dojo, DOMAssistant, Sizzle (congrats for the new site!), Sly, and finally vice-versa project.
Here the summary, from faster to slower:
Dojo 1.3.1 868ms (2 tests failed)
DOMAssistant 880ms (good stuff Robert!)
Now, the good part is that nowadays every selector engine seems to perform truly fast and in whatever browser you like, but the "even better" part is that the experiments file in vice-versa is 1Kb (minified and gzipped) and with just "that Kb" it is possible to use a specific and fast selector engine for FireFox 3.0.
Finally, the experimental CSS to XPath translator cannot support pseudo searches like :hover, :active, or similar, due to the XPath nature, a query language mainly designed for XML where CSS, unless we do not transform it via XSLT, has a complete different meaning from the one used in (x)HTML.
I wonder now who will be able to create better overall performances for FireFox 3.0 ( changing my translator implementation, for example ;) )