In case you haven't seen this yet, take a look at Google Suggest. I hadn't seen it until one of my colleagues pointed me to it this morning - very cool autocomplete feature.
One of the things I love about Google is taking a look at the source of their web pages. You gotta love a company that thinks so much about performance that the html and javascript are heavily optimised to be as fast as possible to download. Also helps obfuscate the code somewhat ;-).
You could find this interesting too:
http://serversideguy.blogspot.com/2004/12/google-suggest-dissected.html