Getting the Most out Of Your Anchor Text: It’s not as Simple as you Think from Slightly Shady SEO goes through how one should vary their anchor text used in their link building campaigns.
As described in the blog post, search engines like Google use to be tolerant of having your anchor text read all the same thing. So if you wanted to rank well for "blue widgets" you would get people to link to you with the anchor text, "blue widgets." That is no longer the case. It is not natural for 99.5% of your links to say the exact same thing about you. The blog post shares an algorithm to help you mix up your anchor text to make it more "natural." Don't you just love when algorithms try to make things more natural?
There is a Sphinn thread where Danny Sullivan equates this to how he and others vary the title tag text to "take advantage of overlap."
Forum discussion at Sphinn.