Google Checking Automated Submissions

I noticed today that Google is now checking for automated web site submissions on the Google add URL form. When submitting a site Google has added an optional field to enter in a string of characters that dynamically displays on each visit to the page in order to distinguish from manual and automated submissions.

This is fairly new as the Google cache of that page from yesterday does not show the extra form.

Recently it was also reported that Google search engine ranking checkers were being presented with the same type of form.

So is Google tightening up the general access to some of it’s services? If Google is getting hit pretty hard with a large number of automated queries, it makes sense. I am curious where else this feature will show up on Google.