Good to know, because I kind of thought that it'd be useful for people who do exactly just that (customer service via Twitter), so this is some validation for me :)

To answer to your question:

* It will scan the tweets of people you are following too, as well as anyone @mention'ing you - respond if there's matches (so it doesn't have to be directed at you)

* Specifically, you said a search stream. I actually had that months ago, then much to my disappointment, after lots of sweat to get it working reliably, I found out if you're auto-responding based on a search, Twitter will not be happy: http://help.twitter.com/forums/26810/entries/76915 so I had to throw out that part ;(