Google has a feature called "Site Search" that does exactly what you are looking for. See http://www.google.com/features.html#sitesearch for details.
Ah, yes I do use that feature but with a few lines of HTML there can be a search bar directly on that page that inserts that code for us. I use it on my phpBB forum. I still have not figured out how to get it in my wiki. Here is the details:
http://www.google.com/faq_freewebsearch.html
I don't quite understand php well enough to get it to generate into the search page. I am still trying however.
You can see it in the footer on my forum here:
http://holographyforum.org/phpBB2/search.php
I would like it to show up when I get a search with no results on my wiki as well. :?
Colin