If you offer printable versions of your site’s content with all the graphics and ads removed you really need to take a couple steps first to make sure you aren’t leaving money on the table.
Back in 2001 SitePoint had an article linked to from Slashdot and thousands of people came to view the article. Good right? Well unfortunately the printable version was linked to, not the normal version, so SitePoint got to deal with all those bandwidth issues associated with a link from Slashdot but they didn’t reap any ad revenue from it. You can read more about it in this old issue of the tribune.
Their solution was to do a referrer check on printer-friendly articles and, if the referrer is not the site itself, to redirect to the normal version of the article. This is something I recommend everyone who offers printer-friendly articles to do.
Likewise you should block such pages from search engines as well, even if you choose not to do the redirect. The last thing you want is for search engines to start ignoring your normal articles in favor of your printable ones, you don’t want to compete with yourself and there is also the possibility (though unlikely in this case) of getting in trouble for duplicate content.
What I like to do is put all files I want to block from search engines, such as author email forms, redirect scripts, and yes printable article scripts, and put them in a directory that I then ban with robots.txt. I think this is the easiest method to make sure those files don’t show in search results.
This same concept can also be applied in a way to forums. For instance vBulletin has an archive feature that basically creates printable versions of your forum threads. The idea behind the archive is that it is a way to get more of your forum indexed in search engines because the pages are bare-boned and do not use query strings in the URL. Unfortunately the last thing you want is someone finding your site through the archive. There is nothing on archive pages to entice someone to join, and there aren’t any advertisements either of course. The truth of the matter is that the archive isn’t even needed, so long as you disable session IDs for search engines, something vBulletin now does by default, you don’t need the archive feature for search engines to fully crawl your forum.
Printable versions are a great tool and you should include them, just make sure they aren’t competing for page views with your normal pages.