I was doing some keyword research for a site the other day and ran into a page from Google based on my searches and looking for SERP results:
There are tools out there that can do this, but we hadn’t finished our signup process for one of the better tools out there. Anyway, it looks like my constant searching (for similar terms probably) and paginating looking for our placement caused Google to notice. Their message:
Our systems have detected unusual traffic from your computer network. This page checks to see if it’s really you sending the requests, an not a robot.”
So apparently Google will get wise to the number of queries you make, the types of queries, and how you use the results. Personally, I have no problem with this. After typing in the Captcha phrase (I hate Captcha by the way), they return you to the next results page.