Google can show you some basic WHOIS information for a domain if you search using the following format whois domain.com. As an example, you can see the whois information for google.com by performing the following search: whois google.com.
If you click on the title for this you'll be redirected to a domaintools page where you can see more details about the domain. This feature is not something new, Google introduced it back in 2004, but it was turned off soon after that because the sites Google used for WHOIS searches were infringing Netsol policies.
As you can see they are using the whois information provided by domaintools.com, because querying whois themselves would probably require much more investments from their side. Useful would be if in the future they'll add also the registrant information in this summary (which they'll probably do).
Anyway, glad they brought this WHOIS search back because this allows to quickly search and see if a domain is taken or not - if the domain is not taken you'll get the "Your search - whois domain.com - did not match any documents", otherwise you'll get the WHOIS summary.