Well I have pages on domains that are not in the SE's, I have the Google toolbar running in Firefox and can honestly say that I have never experienced domains or sites showing up in the SE's index unless I submitted them via link exchanges, blogs, etc.
But yes, putting in the robots.txt should block them...if they listen. Not all spiders follow this protocal and will still follow any and all links it can find on your domain. The same applies to the noindex,nofollow tags.
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