Here's an interesting article from Site Reference.
Basically, it claims that Google are now targeting blog Networks:
"The blog networks are about to die off faster than the dinosaurs. Yesterday, Build My Rank, one of the top blog networks, was obliterated from the web after Google de-indexed its thousands of blogs. Every blog that belonged to the network along with every website that received links will feel the sting of this one.
Some SEOs thought blog networks were sound marketing strategies, but Google thinks otherwise. If you belong to a blog network, you are essentially paying for links and you know how Google looks down on paid links!"
Google Takes Down a Popular Blog Network | Site Reference
I'm guessing sites like build my rank are openly trying to manipulate Google, this is what build my rank says on their site:
"...And since the search engines love to see links from many different sources, we host these domains over many different Class C IP addresses. All precautions are taken to minimize any footprints that the search engines might use to devalue the network
