It's been speculated that G could perhaps query whois data to determine which URL's are owned by whom to determine if backlinks are legit or forged, and the topic has been raised regarding backlinks between sites on the same class C space, but how about NIC card MAC addresses (the hardcoded identifier that is unique to every network card - like a fingerprint).
Could G have a field in the database that correlates each URL to a MAC address? For example my web server has 64 IP's and I have a few websites that point to each other, the IP's are different and I space them about 20 IPís apart but of course the MAC is identical (of course you could run 4 NIC's each with 16 IP's to thwart this).
Now I know you're saying "but they're all on the same class C", yes but can you (or Google) really say for sure that just because 2 websites are on the same class C that they are owned by the same person? No, not for certain you can't. But if 2 websites are on the same MAC address then they are definitely on the same machine, which is about a 99.9% chance that they are owned by the same person (the other 0.1% would be reserved for that 1 in a million chance that a web hosting company happens to have 2 websites on the same server owned by 2 different individuals who by chance traded links).
Of course this is purely speculation but I am curious to hear the input of others on this subject.