Hal Varian argues in the latest post from Google's main blog that "if you look at Google's business, the competition is only a click away. Users can trivially switch search."
While it may seem easy to change your search engine, the reality is that it's difficult. Google is the default search engine in Firefox, Opera, Safari and it becomes the default search engine if you install any Google software and stick with the default options. Google is the default search engine in many people's minds and it's also a name synonymous with searching on the web. Google is the default homepage in Firefox and many people choose it because it loads fast. Google replaces the address bar and it's no longer a web site, it's part of the browser.
Because it's deceivingly simple, people don't treat Google as any other web site. Google is a core feature of their browsers and don't realize they could change it. Even when they find out that Google could be replaced with Yahoo, Live Search, Ask.com, people don't change it because they've already created a connection with Google, learned its tricks and accepted its flaws.
You first accept Google as a browser feature, start to rely on it for things that are important to you. When you learn about possible replacements, you'll start to judge them using Google as a standard and they'll mostly likely fail.
Google camouflaged into a browser feature, into a word, into the default gateway to to the web.