I have signed up(paid) for Google site search. They have me a url of a sort of web service where I can send a query to it, it searches my site, and it returns XML of the sea
For those coming across this now, the AJAX Search API may be what you want: http://code.google.com/apis/ajaxsearch/documentation/
EDIT: Actually, upon further review, that may not hook in with the site search...