While you may be tempted to go through all your links and add nofollow tags to every external link, you may want to consider a few things...
I wouldn't be at all surprised if the engines incorporate some type of red flag concerning sites that have the nofollow tag on all their links.
The purpose of the nofollow tag, from the engines' perspective, is to reduce the spam being submitted aumotacially to blogs in an effort to increase links and PR.
The nofollow tag is not intended to "hoard" PR and I'll be a coke that the engines will, at some time, look at sites abusing the nofollow tag and penalize them in some way.
IMO, it's unethical to use the RSS pheed of others for content, then add a nofollow tag to the links...
Instead, use Followz to:
1. Do unto others...If a link exchange partner slaps you with a nofollow, I don't see any reason not to give it "right back at 'em".
2. For affiliate programs - If you have affiliate links, there's no moral or ethical reason to pass your PR on, and you may as well preserve it for your own sites.
And remember, nofollow doesn't not mean the page won't be spidered and indexed, and doesn't offer any "protection" against being included in the engines. It just tells the engines to "don't count this link" for Google PR and engine link count totals.
-Boom boom boom boom.