I'm not so naive to think that people aren't already doing this all the time, but how is that the SaaS products mentioned in other comments are able to (and/or confident they will continue to be able to) avoid getting a C&D letter from Google? Isn't this a violation of their ToS?
Even using a supported API--which for Google seems to be limited to 100 queries per day--the search engines are officially OK with the submitting queries for the sole purpose of monitoring rankings?
(Looking at Google's new-ish terms of service, I see they are much vaguer about this now. E.g., "Don’t misuse our Services. For example, don’t interfere with our Services or try to access them using a method other than the interface and the instructions that we provide." " rel="nofollow">http://www.google.com/intl/en/policies/terms/>)