I'm sure you've all seen these: large groups of blogs organized by topic that allow you to place relevant posts with backlinks. Similar to article directories, except each post only gets published once. Some are paid, some free. The best ones require 100% original posts, limit backlinks to 1 per post, and the content of the posts and links must be relevant to both the blog and the receiver of the link. These have produced excellent results for a while now. The trouble is, I've recently heard rumors that Google is cracking down on these. Does anyone have any good information about this?