There are so many ways Google knows about related sites, without working hard or being clever. Link graphs, hosting, etc.

It's a popular rumor that Google can access private whois data but that's not how the whois system works. If you register at a registrar providing whois privacy, no one has access to that data except that registrar. Unless they release it, it stays private. That doesn't address whois history, past owners and their registrars, or sloppiness/negligence on the registrar's part etc.

I've asked specific questions on this issue at GoDaddy and Moniker numerous times. GoDaddy reps didn't know the answers sometimes, and sometimes gave me contradictory  answers. GoDaddy also used to keep DomainsByProxy as a separate business from GoDaddy Registrar, which caused further confusion.

Moniker has always responded consistently and specifically. If you buy privacy protection, they do not release the WHOIS data.

looks like I can't edit my own comment anymore. I wanted to add that all registrars (including Moniker) should admit (as Moniker did to me) that they will follow the rules and the law about private whois data (law enforcement, subpoena, etc).  Lucky for us Google is not yet the US government nor the police.

1) No way if you don't link your own sites between each other, use different server, different ip!2) No way if you don't use Google toolbar, Google account, or any other Google services.

As long as you are following the rules for each of the sites I really dont think it matters if they do. However to the question I have always followed the rules of 10. No more then 10 websites can be linked togeather. They must support eachother, however be completely unique from eachother, and seperate ip calsses is always a good thing. Also make sure you spread the launch times apart within each network as much as possible.

using private whois goes a long way, other ways which may work but can be very time consuming is to host sites on different ip c-classes, use different domain registrars and mix up whois registration details (use family and friends contact and address details).