This is always an interesting topic so I thought I'd jump on over and leave my 2 cents
- The quickest way to get it out of the index is to use the "noindex" x-robots-tag (as Alan mentioned a bit back). These HTTP headers are understood by all of the major search engines (as far as I know).
- Another way to get it removed is to block it with the robots.txt using a "disallow" directive. In a situation like that, we'll still read your robots.txt (because we have to check it anyway), but it'll generally take a bit of time for it to drop out of the index (because "disallow" blocks crawling, but not indexing). A way to speed that up is to use the urgent URL removal tool within your Google Webmaster Tools account.
- That said, it's not like your robots.txt file is going to rank for anything really interesting, so I wouldn't worry about it being indexed.
For what it's worth, the WMW robots.txt is using cloaking. You can see that by looking at the cached file:http://www.webmaster....com/robots.txt
I would certainly NOT recommend doing that. Not only is it against our Webmaster Guidelines, but if you have the slightest hickup in the cloaking code you'll suddenly be serving the wrong robots.txt file. Don't play with your robots.txt file