Chris Sells points to a blog post in which someone undergoes two days of interviews for a contracting position at Google.
The poster mentions a Google interview question that refers to the famous birthday paradox. However, the poster seems to have recall the interview question incorrectly, as it has an trivial, uninteresting solution. The poster stated that the birthday of one of 9 people in the parties must match his, rather than anyone else at the party. Even then there would only be a twelve percent chance of getting a matching birthday between any two persons. The interviewee apparently struggled a bit through the problem, before subsequently being offered a job many weeks later (apparently, not the first or second choice).
Google seems to have topped Microsoft in its approach of recruiting and interviewing eggheads. For example, the search engine company, in the past, has placed in billboard ads complex mathematical puzzles, which when solved directed the person to a website.
Contrary to popular belief, I always felt that Microsoft interview questions were actually rather straightforward for seasoned, talented software developers; the inability to correctly answer a question is a flashing red sign. Google, on the other hand, seems to screen for higher-level mathematical reasoning. This explains much of my special affinity for a company founded by two PhD students.
Despite this, I am not sure that screening applicants through puzzles should be the only approach. (Google, by the way, also puts equal emphasis on ability to work on team.)