Google penalty checklist

This thread is touching on higher-level issues, but I'm more interested in what's going on with lgn's sites. The first thing to know is that most of the time when a site disappears from Google's index, it's due to the site in question. I know it's easy to jump to the conclusion that Google is somehow punishing a site, but that's probably not right just going by the numbers. Here's the checklist I'd suggest:

1. I'd check whether the ISP has been up the entire time.
2. I'd also check using
site:yourdomainname.com -asdfasdf
to see if we actually do have any urls from your site. Our rankings can change, and just because you don't show up for a search doesn't mean that we don't have your pages.
3. Triple-check your robots.txt. If it doesn't exist, I'd make it just to be safe. The very last company that I worked with was worried that they had penalties. Nope, they were running JRun, and JRun was giving a 500 error when spiders tried to fetch robots.txt. We would try several times to re-fetch the robots.txt, always fail with a 500 error, and so we didn't crawl the site to be safe.
4. Communicate with Google. I checked the spam report queue and didn't see any messages with your nickname of "lgn" for example. But the preferred ways to write would be to help@google.com. If you suspect that some staff member did spam on your site, or you get back the email from Google that says we were unable to assign PageRank to your site, then I would play it safe and write to webmaster [at] google.com with the subject line "reinclusion request." Describe your situation (when did this happen? Is your site completely gone or just not ranking how you expect? Did you do anything like hidden text, and what steps have you taken since, etc.).

If you've gone down that checklist already, please let me know. If our process isn't working as it should, then we'll want to see what changes we can make. Without more specifics, I can't debug the situation--you don't list your site in your profile, for example.