The problem with those reports is that they monitor the availability of 1 domain. A company I worked for in the past shows up at 99.99% with zero downtime this year. I know this to be actually incorrect. It's not even correct for the domain that webhostingstuff.com is monitoring. It was 99.9%+ but not 100% for 2007. The other problem there is the frequency that the domain is monitored. If it's once per hour (which I believe it is for webhostingstuff) then I have a 59 minute window to reboot a server and it not show up on the downtime report.

Artashes, in answer to your question about uptime for updating servers... load balancing would be the way as you could plan to reboot 1 server while the other(s) are working away and then repeat through each of the servers.