Google Instant Search = Instant Echo Chamber
How does Google Instant Search contribute to the Echo Chamber? Well anyone who has bothered to look through keyword information on their website will see that people find sites through a bewildering array of queries. Some might even say much of it is accidental, but looking over these lists gives a wonderful window onto how your content is found and perceived by others. How often do we get to commit such telepathy with our followers? Rarely. Yet, Instant Search will substitute popular searches for those individually created. More people will be driven off the back roads search trails and onto the superhighways that lead to whomever controls the first few search results connected to the Instant Searches Google is recommending at the time.
Interesting implications for SEO and AdWords bidding.
Update (2010-09-11): Wade Roush has some theories.
It's an interesting theory, but I don't believe it will have such a dramatic impact, or any impact at all.
You could argue that it will in fact instead encourage to search outside the beaten path, since you get instant results. It makes the user want to try more things, instead of just going with the first set of search results: without instant search, there is some friction to try another search, so you are less likely to do it, and more likely to just settle on that first search.
Either way, time will tell :-)