You write: "anytime a new comment is posted the item would be updated".
Do you also want to update the date or only the content?
I’m sorting items by date in my newsreader, and so for me it would be necessary also to update the date. Without that, I probably wouldn’t notice the update of the item.

Would it be easier to have a tool (or a site) that subscribes to comment feeds and selects relevant posts out of these. When posting an comment somewhere, you could ping the tool with an bookmarklet (just like ping-o-matic) providing the URL of the article that received your comment.

Of course, this assumes that all sites provide comments as RSS feeds, either seperate or within the main feed.

Alternative: A tool on "my" server (small extension of Adelberts suggestion), that
1. aggregates the (sometimes) already existing RSS-Feeds for comment threads
2. includes a watchdog-process, which is able to detect changes in the public HTML of the post/comment page and RSSifies these changes and aggregates them too.

Seems to me that this would be
1. harder to develop (obviously)
2. easier to roll out (nobody else would have to change his code to suit me …)

I’ve been thinking about comment authentication and tracking in this post: http://blog.monstuff.com/archives/000153.html

In the second "auth by url" solution that I proposed, I detailled how to use the auth interaction to set up a shared secret between the site I use to authenticate and the site where I comment, for later notifications.

Also, about Markus’ comment, it seems like all blogs that support pinging, could be modified to ping when comments are posted. But I agree that that change would not be immediate…

Related: Helge, why donīt you have trackback enabled here?

thinking about it: you might be able to use zfeeder if the comment section has a unique rss feed (like described above). then rss zfeeder and boom… (it’s just a quick thought, might need some tweaking)