Interesting letter type responses from Google guys on the way Google understands how it handles If Modified since and other server errors.

"Hello Jimmy, Let's pretend there are no anachronisms in your letter, and get to the meat of the matter. Firstly, let's look at links coming from other sites. Obviously, these can be a great source of traffic, and you don't want visitors presented with an unfriendly 'Page not found' message. So, you can harness the power of the mighty redirect. There are two types of redirect—301 and 302. Actually, there are lots more, but these are the two we'll concern ourselves with now. Just like 404, 301 and 302 are different types of responses codes you can send to users and search engine crawlers. They're both redirects, but a 301 is permanent and a 302 is temporary. A 301 redirect tells me that whatever this page used to be, now it lives somewhere else. This is perfect for when you're re-organising your site, and also helps with links from offsite. Whenever I see a 301, I'll update all references to that old page with the new one you've told me about. Isn't that easy? If you don't know where to begin with redirects, let me get you started. It depends on your webserver, but here are some searches that may be helpful:Apache: http://www.google.com/search?q=301+redirect+apacheIIS: http://www.google.com/search?q=301+redirect+iisYou can also check your manual, or the README files that came with your server. As an alternative to a redirect, you can email the webmaster of the site linking to you and ask them to update their link. Not sure what sites are linking to you? Don't despair - my human co-workers have made that easy to figure out. In the "Links" portion of Webmaster Tools, you can enter a specific URL on your site to determine who's linking to it. My human co-workers also just released a tool which shows URLs linking to non-existent pages on your site. You can read more about that here.Yours informationally,"

For more information read the blog here, http://googlewebmastercentral.blogspot.com/2008/11/date-with-googlebot-part-ii-http-status.html