How to put comments in RSS feed?

A few days back I subscribed to the RSS feed of Metroblogging Mumbai and I was pretty impressed that they had done a good bit by inserting comments into the feed itself.

Metblogging Comment

So when I read the post I can see the comments right there from within my Google Reader. Now, what would be interesting is to know if the comments get updated in the feed.

I tried to do a Google search and see if I can find a plugin to do the same with my blog feed. I haven’t been lucky yet. Let me know if you know how to do this.