You know how Google has everybody afraid of links? People are also afraid to link to their own stuff in certain ways, and Google's Webmaster Help video today pretty much indicates that this is with good reason.
If you have a bunch of domains, you need to be careful about linking them to each other, because Google's not a fan. Matt Cutts took on the following question in the video:
Should a customer with 20 domain names link it all together or not, and if he links it should he add nofollow to the links not to pass PageRank?
"Well first off, why do you have 20 domain names?" Cutts begins. "You know, if it's all 'CheapOnlineCasinos' or "MedicalMalpracticeInOhio,' you know, that sort of stuff, having 20 domain names there can look pretty spammy, and I probably would not link them all together. On the other hand, if you have 20 domain names and they're all versions of your domain in different countries, right - Google.co.za, Google.fr, Google.de - that sort of thing, then it can make a lot of sense to have some way to get to one version of the domain to a different version."
"But even then," he adds. "I probably wouldn't link all the domains even in the footer, all by themselves, because that's a little bit strange. I'd probably have one link to a country locator page, which might even be on domain.com, and you might have flags or something like that, so there are ways to get to those other domains. And as long as there's a good way for users to get there, then search engines will be able to follow those links as well. Just make sure that they're normal static HTML links, and we'll be able to follow, and the PageRank will flow, and all of that sort of thing. So if there's a really good reason for users to do it - maybe you could have a dropdown where you could pick your country or something like that - then it might make sense."
"But having the country top-level domains is one of the only areas where I can think of where you'd really need to have twenty different domains," says Cutts. "In theory, you might have a blog network, but even then, you know, I've seen very large blog networks, and you've got that footer at the bottom that has a lot of unrelated domains, and at some point it gets pretty big. Even then, you'd probably only have like ten domains, and maybe a few posts on each domain that are linking to each other, so at the point where you have 20, unless there's a really good reason, I would be a little bit leery of just doing some massive cross-linking scheme between all of them."
So it appears that even if you're not trying to "scheme" per se, Google might view it as a scheme, if you link your own web properties together in a way that it doesn't like. Of course, there's always nofollow.