Michael Gray has written an interesting, if not mildly paranoid blog post to explain how Google could potentially identify website relationships via XFN links. The idea that Google would use this information to penalize publishers doesn't seem too far fetched, as Google has been known to pull some fairly shady shenanigans on not so rare occasion.
The first video clip Mr. Gray has posted is extremely informative. If you don't have time to read the entire post, I'd recommend at least watching the clip in order to obtain a better understanding of how XFN relationships work.
Excerpt from the blog post:
Comparing notes with a few people, the two biggest discovery methods are webmaster central and Google analytics. So anything that’s in either of those two areas is likely to to be shown. That includes actual client sites, potential client sites or anyone else who shared/granted you access along the way. Makes you wonder about that crazy guy who gave you access and you blew off because he had a shady backlink profile. Does google count that against you?
Does Google Know What Websites You Own
What is your take on this? Are you currently doing anything to dissociate your sites?