Feeds don't count as duplicate content at least in my experience, but I know how it feels to be paranoid about it. With all the horror stories you hear, you start thinking "Any wrong move and you're gone forever."
I have a PR3 blog that I listed with most of the blog directories. Every time I post I ping
http://pingomatic.com.
Last week I did a test on a keyphrase. Within 24 hours, I was number two for the phrase. That was the index page of my blog. Since then I have dropped because new entries pushed that one further down the page, but the archived entry is now in the top 20.
A lot of the sites that pingomatic pings get at least their index pages hit by the Googlebot every day.
And that means if you write an entry and ping these sites through pingomatic, you have instantly created almost a dozen links back to your site for that day.
With the keyword I tested, I made sure it was mentioned in the title of the entry and high in the entry of the blog itself, considering that my feed will only hold the title and the first few sentences of the entry.
Hopefully I didn't give too much away there. LOL. But just to let you know, the results surprised me a lot.