Oh, there's a good way to make external links nofollow, without using the nofollow attribute (rel="nofollow"), or in a position when you're unable to manually set it. It's also handy if you really want to make sure that it's nofollow - like when linking to a suspect site or something of the sort. It's not exactly relevant to your question, but it might help somebody else reading this thread.
1. Let's say you're linking to Google. Create a directory in your site called 'out' or 'outgoing' or 'links,' anything like that to help yourself maintain organization. Then create a file in that directory called google.php -- then put this (and only this) in the file:
2. Now, to make that entire directory nofollow, edit your robots.txt file and add:
header( 'Location: http://www.google.com' ) ;
Disallow: /out/ (or whatever you called your directory)
3. Then, whenever you need to make another outgoing link, you can just create another file using the code above!
Clarification: When someone is taken to yoursite.com/out/google.php they will be redirected immediately to Google's website. However, search engines will not follow the link, ensuring that it's nofollow and that none of your PR will be spread. It also helps if you're trying to hide the fact that you're linking to that site.