Is it possible that Google - being a domain registrar - allows them to see all the registration data for any domain? I'd like to know - not for linking reasons, but for owning several industry-related domains. If you've registered several similar 'XYZ Service in Springfield' domains, and have several corresponding sites - each with unique content and design - and with different hosting providers, would Google be able to see that the same registration information exists on these domains, even with whois privacy on the domains? Could this result in indexing penalties (since the supposed different sites/companies are registered to the same name). We know that Google wants different individuals/companies represented in SERPs, so what do you think?
Google cannot see behind privacy registration - however, they could have access to historical ownership (i.e. ownership of the domain BEFORE privacy protection could be recorded). There are also some who believe (like Aaron Wall) that privacy protection can raise a red flag in cases of interlinking sites. He also believes there could also be other signals used (all domains have the same registrar). I personally don't believe you would have a problem with a few domains. It is easier to build backlinks for a single site vs. a number of sites trying to rank for the same keyword, but I personally have had similar sites that have taken 5 of the first ten results for a moderately competitive keyword and I am sure that Google could make the ownership association if they wanted to. The more thin or spammy the sites, the more risk.