Matt Marshall has a nice writeup on Powerset, the natural-language search engine that just closed a big first round of funding from Foundation Capital and the Founder's Fund. Big congrats to Barney, Steve and the whole team -- from what we've seen thus far, they are on to something big (Barney is a friend and member of our Advisory Board at Krillion, I should add).
As to the larger issue, what is the "holy grail" of search? Is it natural-language search, or something even more futuristic?
In a word, no. Natural language search, PageRank, hand-surfed directory, etc., it simply doesn't matter what the means are so long as the end is achieved -- don't get between the user and what they are looking for.
How do you know when you've found the Holy Grail of search? I'll rely on Potter Stewart for this one and simply say I'll know it when I see it. A sure sign of its imminent arrival will be when you can go to a search engine (any search engine) and type in "cheapest flight from SFO-JFK" and know with any degree of certainty that you truly found the lowest fare. Travel has more advertising dollars and more consumer usage than any other online industry, so if they haven't solved that yet it will be a while before other queries yield super-relevant results.
Check out the results from Google and Yahoo! and you'll see that search still has a long way to go.