I agree with Aryeh in principle; when you're updating the suggestions on every keypress, extra processing and DOM manipulation at the Javascript level would be good to avoid.
As far as Aryeh's lack of other use-cases for comboboxes in general, a good place to use one would be for inline additions of keywords/metadata tags in a web application or on a web site. It would allow for trivial entry of new metadata tags using existing ones if available or any arbitrary new one if required. I've also built apps that have used autocompletes for things like email addresses/usernames in private messaging fields and other similar situations. Unlike search applications, these fields are often used inline within other fields/containers. I would hope that Maciej's example (using the standard OS X combo box) would permit significant CSS styling to fit that scenario. —Kit
