On a SES Session Summary we see the following:

Q: I have a client that has a great site, lots of links but the anchor text being used throughout the web is the same.
A: Matt said that is very unnatural. Most natural links are not 100% one exact phrase to the site. It won’t hurt you, there is no OOP, but all the links might be devalued.
Tim agreed with Matt on it being unnatural.

K. Let’s consider the following situations:

• I have a web services company (services from hosting to web design) and I put my link on all my clients
• I have a cool freeware PHP script and I have included a link to me in it.

How unnatural are those situations? To mee it seems pretty normal….but maybe it’s just me

Another one:

Q: Reciprocal links; we have them now, we have plans to do more, what should I do? There are 20 of them links.
A: Matt said here is my rule of thumb, pretend you are my competitor, what would they think of it? Plenty of people have reciprocal links but if its excessive, then you need to be careful. Editorial given links and independent links are best.
Danny then asked 4 people in the audience to point to each other and then asked several to point at each other.
Matt said if you go into “graph theory” you have a “clique”, that clique is when everyone in a network is pointing at each other, that is not natural.

K. I have a blog. I read other blogs. I find some interesting and I quote them or link to their main page. Analyzing logs or seeing comments, the others bloggers decide quoting me or linking to me. I have not beg for links, I have not payed for them. Is natural

In other order. I know that at the examples above there are hundreds of illegal counter-examples. My point was:
“The search engines could tweak their algos in order to spot natural/unnatural within the same behavior?”

Perhaps the answer is no…or not now