I'm guessing a site like stack overflow doesn't keep an html file around for every question ever asked. Instead, server-side code creates the page every time a question is clicked on(I think). Is it possible for search engines to index every quesiton on Stack Overflow, or would a page-per-question need to be kept in the directory so the search engine can crawl it?
You might be interested by the Dynamic URLs vs. static URLs post on the Official Google Webmaster Central Blog.