I recently did a 301 redirect to have the search engines ignore one of our sites (call it www.powered-by-our-other-site.com) but it caused problems with some of our client's sites. It looks like I really didn't want a redirect, I probably just wanted an ignore signal in robot.txt, right?
I removed the redirect a few days back and our main site has dropped a few spots in the rankings and the "powered by" is showing up again. I will find the pages that link to the "powered by" page and no follow those links. I am still wondering about robot.txt.