It's an engineering and support facility. Looks like they'll be working on analyzing software and networks for vulnerabilities.
I checked over at CareerBuilder [careerbuilder.com] and found a bunch of fresh job openings like this:
Google is looking for Network Security expert to help provide security for our networks and systems.
Google is looking for world-class Security Engineers to help us ensure that our software and infrastructure is designed and implemented to the highest security standards. If you enjoy analyzing software designs and implementations from a security perspective, and you are an expert at discovering subtle security issues that appear under unexpected threat scenarios...
Those listings are more specific than the jobs listed [google.com] on Google's website.
Six hundred people is a lot. I wonder if this is in preparation for working on current products and initiatives or in preparation for something really big.