This announcement was posted today to LiveJournal's "LJ-Biz" community. This is where LiveJournal offers explanations for its business decisions, after those decisions have taken place. Apparently this hasn't been released to the general public yet, because it has only appeared on this LJ community and not in a public announcement yet:
http://community.livejournal.com/lj_biz/242602.html?nc=85&style=mine In a nutshell: LiveJournal is going to be applying cookies to random blogs on its system. The cookies will be completely random, which means no one is supposed to know who has a cookie on his blog. This is being done on an opt-out basis, which means you can choose not to have it...if you know about it in the first place.
LiveJournal's users are acting in the expected manner: they're going bat-guano over it. Check out the comments and see for yourself.