why do you want to do that? one in the main folder does the same thing....and I think the spiders only look for the robots file in the root directory anyway
i've one in the root. i'm using a script in folder which comes with robots.txt. so i have to keep robots.txt file there also otherwise i've to copy it to the root by changing all its urls. I checked it out and came to know robots.txt in folder also works as fine as roots........