My goal would be for the x-cart to update the rss.xml file everytime you use the newsletter module, and make changes to the the postings. I have no interest in bringing other external feeds for display.
It could be as simple as adding in a function call within the newsletter code, that wrote out the XML file. No display of the feed content anywhere, except possibly a button on the side menu that links to the XML file for users to subscribe with.
I liken this ability to the way X-cart has been updated with the ability to create and send Froogle updates automatically.
Like jdiehl says, I don't want to rely on external services like the blogger.coms to do the work, I guess I'm picky that way.
Still, you can use the aggro sites to send your feed on for more folks to see. No harm in that.
