I've seen a lot of questions lately about robots.txt files and Googlebot's behavior. Last week at SES, I spoke on a new panel called the Bot Obedience course. And a few days ago, some other Googlers and I fielded questions on the WebmasterWorld forums. Here are some of the questions we got:

If my site is down for maintenance, how can I tell Googlebot to come back later rather than to index the "down for maintenance" page?
You should configure your server to return a status of 503 (network unavailable) rather than 200 (successful). That lets Googlebot know to try the pages again later.

What should I do if Googlebot is crawling my site too much?
You can contact us -- we'll work with you to make sure we don't overwhelm your server's bandwidth. We're experimenting with a feature in our webmaster tools for you to provide input on your crawl rate, and have gotten great feedback so far, so we hope to offer it to everyone soon.

Is it better to use the meta robots tag or a robots.txt file?
Googlebot obeys either, but meta tags apply to single pages only. If you have a number of pages you want to exclude from crawling, you can structure your site in such a way that you can easily use a robots.txt file to block those pages (for instance, put the pages into a single directory).

If my robots.txt file contains a directive for all bots as well as a specific directive for Googlebot, how does Googlebot interpret the line addressed to all bots?
If your robots.txt file contains a generic or weak directive plus a directive specifically for Googlebot, Googlebot obeys the lines specifically directed at it.

For instance, for this robots.txt file:
User-agent: *
Disallow: /

User-agent: Googlebot
Disallow: /cgi-bin/
Googlebot will crawl everything in the site other than pages in the cgi-bin directory.

For this robots.txt file:
User-agent: *
Disallow: /
Googlebot won't crawl any pages of the site.

If you're not sure how Googlebot will interpret your robots.txt file, you can use our robots.txt analysis tool to test it. You can also test how Googlebot will interpret changes to the file.

For complete information on how Googlebot and Google's other user agents treat robots.txt files, see our webmaster help center.