Google’s head of search spam Matt Cutts answered a question on that topic in a recent video which asked “If I have 20 domains, should I link them all together?”
In short, Matt Cutts said that it is very unlikely that a webmaster would have 20 websites on a similar topic and for those sites not to be somewhat spammy. “First off, why do you have 20 domain names,” Matt joked. Matt added, “if it is all, you know, cheap-online-casinos or medical-malpractice-in-ohio or that sort of stuff… having 20 domains there can look pretty spammy.”
When would it not be spammy to do so? When you have 20 domain names but they are all localized versions of your site and you are linking to them, then that would be okay. But even in this case, Matt said you shouldn’t link to all these domain names in the footer — instead have flags or a drop down to access them.
Matt Cutts then talks about blog networks doing it, but advices against it.
At the very end of the video, Matt says it would be a “cross linking scheme” to link all of these sites together, unless there was a “very good reason” to do so.
Here is the video: