Thanks for the nice long article Damien.
There's another NEW reason people are being dropped that has recently come to light. Well it's been going on for awhile but people are just starting to talk about it.
I've been involved a little in the blog that's exposing the problem. You guys need to take the "proxie test" like I did and find out if you could be affected. It's pretty scary.
Google Proxy Hacking: How A Third Party Can Remove Your Site From Google SERPs