Review my web app, CraigUpdate.com
Davertron: I think the site is OK, but I get this same general functionality from doing a search on craigslist and then adding that search as an RSS feed.  While I don't get INSTANT updates, I'm in my RSS reader often enough during the day that I usually see things quickly enough.  Still, not a bad idea, and if you're really looking to be the first person to see a new post, I can see how this would come in really handy.