Planet13 - 6:42 pm on Jun 21, 2011 (gmt 0)
While your sites have been successful so far, i can't really recommend all the above methods.
If one is to search these forums for over optimization penalty they would see that several people who have done those types of link building have been hit with a recovery time of between 3 months and 2.5 years.
When your site jumps up the SERPs quickly, you leave yourself open to manual reviews, and a site with a link building profile like the one mentioned above would, in my humble opinion, be very susceptible to a penalty.
A few months back there was a poster who advocated something along the lines of what you suggested. Just about two weeks ago, he was back on the google forum lamenting receiving a -50 penalty across several sites.
For what it's worth.