This topic comes up frequently enough that it's been labelled a "classic" Google problem. This thread below has some good discussion and also includes a link to a WebmasterWorld site search on the Google message....
"your query looks similar to automated requests" classic google problem - any solutions yet? [webmasterworld.com...]
Special search operators such as inurl: or intitle: with known footprints for guestbooks and such (link spamming :)) will trigger it. You have to loose the search operators or exact phrases and replace them with regular terms aiming for the same result.
[edited by: tedster at 4:39 am (utc) on Jan. 11, 2008]