this never come to my mind before but seems to be so logical lets imagine google crawler come to your site while you have bad downtime you risk to have too many pages left out and interpreted as inaccessible results thus PR goes down steeply ! it's time for us as publishers to rethink downtime very carefully and care more about hosting
google doesn;t calculate PR or assign ranking based on a single crawl. if you have too many pages that google needs to crawl, then google bot will visit your site often and sometimes several time a day. therefore don;t worry about a single downtime. however, downtime should be avoided as much as possible. if your visitor cannot view your site, they are not going to make you any money
If a site goes offline for more then 10 hours or so, here is what Google recommends.. http://www.seroundtable.com/archives/015171.html >>>"The server is currently unable to handle the request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay. If known, the length of the delay MAY be indicated in a Retry-After header. If no Retry-After is given, the client SHOULD handle the response as it would for a 500 response. Note: The existence of the 503 status code does not imply that a server must use it when becoming overloaded. Some servers may wish to simply refuse the connection."<<<< Thats what a 503 does when the site is down
The worst of impact of the website downtime is NOT PR, it's the business. Loss of business and visitor's confidence is not something that can be measured. I agree that rankings are not based on one or two crawls. Spiders come and go very often and chances are that they will come to a site when it is up. But you never know when the most important buyers will visit your site. If your site is down when that happens, you can be losing a big account. Having the highest uptime is one the most important aspect of web hosting.
The biggest fear is when moving your equipment from one datacenter to another. You better make sure you plan it appropriately and make backups of everything. If you 100% rely on web for money and can afford it, you should go ahead and buy some fail over boxes setup with your exact configurations in case something happens during your move. Screwing up a datacenter move can really hurt your business.