Because of a massive data center outage at my hosting provider yesterday my server was down for 3 hours. I used to have very good and steady positions in the serps for several months in a row. Less than 10 hours after this outage however my positions on Google changed. Between 3 and 5 places down for my most important keywords.
Is it possible that Google reacts this fast on websites that are down or is it a coincidence? Did anyone ever experience the same?
It's possible -- I would personally "don't worry about it" for a week or two -- I had a domain last year set to auto-renew that didn't get renewed due to cc info changes and I didn't realize it until the domain was way into the redemption period, meaning the site "completely disappeared" and did not show for anything it did normally.
Long-story-short, it took less than 2 weeks after renewing it for the rankings to be restored. So, based on my experience, don't worry about it for a week or two -- If you don't "pop back in" where you were in that time-frame, then I'd start to worry a bit more.
From my perspective, you are 100% correct Chris. It seems from previous experience that any outages over some specific amount of time (lick finger & stick in air), say around 6+ hours, hurts like hell your rankings. Just takes a few days to a few weeks (pending the length of the outage) to get back to normal.