herculano - 7:55 am on Jan 11, 2010 (gmt 0)

If you are using php, you can configure php to make the robots.txt executable, afterwhich you can add the following logic:
1. check the user agent
2. if user agent is google bot, msn or other bots you want to allow to read robots.txt as usual but do 3. first.
3. do an ip check and then do a reverse ip check, if indeed the bot is really who it is; it should end as googlebot.com, msn.com or yahoo.net.
ex. gethostbyip check the string then gethostbyname to do a reverse ip check then compare the ips.
4. after both checks are complete and authenticated; you can show the robots.txt accordingly.

I think that should work.. haven't tested it myself though..

hope that helps.