I have an idea about a way to do a blog aggregator. Just an idea, I'll let the more technically knowledgeable people fact check this for viability.
Google Alerts (
http://www.google.com/alerts) can be set up to search blogs for keywords, and send you alerts by email, in Google Reader, or via an RSS feed. It gives you code for an RSS feed.
People who want to have blog posts appear on The Feed add the keyword to the blog post somehow, so Google picks up on it. It has to be a unique keyword. I suggest "rpmnfeed".
People who want to host the feed can just paste in the RSS code on a web page. Then The Feed will be available in multiple places. Or, you can receive the feed in any way Google Alerts serves it up.
In the event of spammers, we change the keyword.
No hosting required. No moderation required. It's distributed and open. And it should be relatively simple to execute... if it works.
Now, I need to find Java or Flash RSS code that works on a Ning page. Ning's native RSS limits the number of articles served.