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is that just to stop engines from crawling certain areas on my site?
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Yes. It tells a bot where it is allowed to go and not allowed to go, most people use it to exclude bad bots and email harvestors.
I don't use a robots.txt file, and I'm not completely experienced about setting one up, but it not very difficult as you can read about it many places on the net and google faqs.
I don't use a robots.txt file because I monitor all my 404 errors coming in, and without a robots.txt file I get a report that includes all the bots and email harvestors that get reported with the 404 errors.
Matt may have such a script for reporting such errors, I am not sure, but it wouldn't solve your problem.
I use Guardian... http://www.xav.com/scripts/guardian/
Getting back to your original question, it may be that Google and some of the other search engines are not including discussion groups in their search, for a number or reasons... too much info out there, too easy to spam the index, not very stable. My discussion group used to be index and crawled all the time. It doesn't anymore. I just did a search for some text in one of my discussions from last year and it isn't in google.
Try going to http://www.webmasterworld.com there are lot of smart guys over there that may give you a more definitive answer.
You could make a "doorway" type page and put keywords there to have the search engines find your info.. some people have mixed feelings about a doorway page.
You could put your info in a framed page, and within the "noframes" tags place your keywords and info there.
I know stuff... but sometimes I just don't know, what I know... :-))