Today, the Selectors API specification has been published as a Last Call Working Draft. Assuming all goes well, and there are no comments requiring substantial changes over the 4 week review period, the spec will go to Candidate Recommendation shortly after, at which point it will remain until there are two interoperable implementations. Although the spec says comments are welcome until 2008-01-06, this is likely to be extended by a couple of weeks due to the delay in publishing.
I need to begin work on a test suite for it very soon so that we can help ensure interoperable implementations with as few bugs as possible. It would be nice to see this implemented in the major browsers soon, although I could only speculate about when each one will.