Out of the box it might not look like Pixie supports RSS of your comments on a particular blog post but using the power of microformats you can offer RSS to your visitors. Lets see how...
First off you will need to pick which post you want to let people subscribe to, I am going to choose one of the first posts in this blog (as it is has quite a few comments on). Next up you need to find a service that converts the hatom microformat into a digestible RSS feed. I recommend using http://xoxotools.ning.com/hatom2rss.php. On this site you will see an input box for your URL, paste it in a press Go!
You will now have a subscribe-able RSS feed for comments on your blog post. You could easily setup a block that provides this functionality automatically... in fact I may do that very soon.
