This is a tad funny, the fact that Matt Cutts, who started a blog a couple months ago at http://www.mattcutts.com/blog/ has yet to post a robots.txt file for the spiders.
I know he has yet to do a lot of things and will get around do it. But hey, I do not even have a robots.txt file for this site. It is obviously not that important if you do not mind spiders crawling your site and you do not care how they crawl it.
But for larger sites, it is always wise, this way you can optimize speed and resources to its max.
Forum discussion on Matt not using the robots.txt file at Search Engine Roundtable Forums.