I have just started using Disqus and looking for "on-topic" sites that use it.  None of my posts have been indexed on non-wordpress sites.  I did some research and found that Disqus uses Javascript to render the comments which googlebot won't be able to read.  However, it appears there are plugins in wordpress that renders the comments into HTML that can be seen when you view source.  Thats probably why you see your comments being indexed on wordpress sites and not the others.

I'm still testing this myself and would like to hear from others on this topic -good or bad.