Natural language understanding is quite relevant for the crawling and indexing part of information retrieval systems and Google is very good at that. Just look at their quite formidable automatic translation software, which is a by-product of their ability to correctly map natural language concepts to strings.
The thing is: People just don't want to converse with a search engine as if it was a human being. Some library / scientific information retrieval systems tried to go in that direction, which resulted in retrieval systems that were just cumbersome to use.
Google nailed the search engine user interface quite some time ago and Peter Norvig is absolutely right when he says that users simply don't want to ask questions when searching. They're much faster at entering keywords relevant to their search intent because they've learned how to efficiently converse with search engines in their 'native' keyword language.
Hence, passing the Turing test is completely irrelevant for search and information retrieval in general. Even in mobile environments where due to the device's constraints a natural language user interface makes a lot more sense than on the desktop, software like Siri more or less is just some gimmick that in most cases is easily outperformed by more traditional input methods. Sure, asking Siri to 'Show me the way to the next whisky bar' might be fun at first but simply entering 'pub' and the name of the town you're in right now is still a lot more efficient. Again, I think Google nailed the user interface part with Google Now for mobile information retrieval as well. I don't want intelligent machines to pretend they're human. I want them to take a back seat and present me with the right information once it becomes relevant.