This has been on the cards for a while, Google is getting better at detecting those blog networks. One of the biggest public networks buildmyrank.com has had a large number of its blogs deindexed by the big G. Why?? Well they were allowing their clients to game the system all too easily. Google has been getting better at detecting these networks and now is taking action. Lots of smaller networks have been hit recently, but this is the biggest network to be tracked down and punished.
You can read all about this on buildmyrank.com’s offical statement.
No I didn’t use them and don’t use any public networks. Whilst the rewards are there its also quite easy to detect and hence quite risky. Stick to the more traditional methods guys and you should be ok.