I found the answer in a simple plugin called ‘Nofollow Blogroll SEO’:
http://wordpress.org/extend/plugins/nofollow-blogroll-seo/
This plugin has no settings. It simply adds ‘rel=nofollow’ to all your blogroll links, unless the blogroll is being shown on the home page. Perfect!
Here’s some information from Google about how they handle ‘nofollow’ links:
http://support.google.com/webmasters/bin/answer.py?hl=en&answer=96569