When I joined ThoughtWorks, almost 2.5 years ago, I had a conversation with Paul Hammant where he said he always googles someone before he interviews them. At the time it was a good idea, but now I think it's a great idea.
It's fairly common, in my experience interviewing TW candidates, to list every technology a person has ever touched. If they've done a 'hello world' in IO, you'll find IO on their resume.
Contrast the above situation with what you find on a person's blog or in their emails to mailing lists. For example, I have done 'hello world' in IO, but there's not a single entry on the web that links me to IO. Conversely, a glance at my blog shows I've been doing Ruby/Rails full time for more than a year at this point and Agile for almost 3.
A quick search can save you from a lot of tracer bullet questions. Your first question in an interview usually needs to be very high level, to see where the candidate skills are at. The next question may be very detailed, to see how deep the candidate is. But, a simple search could have revealed the same information without the common interview dance: If you ask a high level question, you usually get a high level answer, despite the fact that the candidate may have a very deep understanding of the topic.
Of course, this wont work on all occasions. Searching for Michael Johnson isn't likely to produce relevant information, nor will searching for someone who never publishes anything on the web. But, even if it works 50% of the time, it's better than having to ask the same broad questions at the beginning of every interview.
Also, if this practice were more widely adopted, having a blog may become an advantage when searching for a job in the future. Personally, I'd prefer someone be able to size me up before an interview. I don't enjoy tracer bullet interview questions any more than the person asking them.