Quoting the response to my eMail that I wrote to the Disqus Heldpesk:
Trackbacks are currently not being included as comments, we’re sorry for the inconvenience. We’re looking into right now and will be addressed in one of our upcoming updates. You can catch these updates on our blog at http://blog.disqus.com
Ryan (end of quote)
It seems as if we have to wait a little longer for that feature.
[Update from 2010-07-09]
Until they have found a method to include ping-/trackbacks, I will use a little workaround based on the WordPress database.
[Update from 2011-01-03]
It seems like the Disqus Comment System had a major overhaul, which also means that it now shows ping-/trackbacks that are stored in the WordPress comments table. Nice, but I’ll still stick with my aforementioned workaround, since I do not like the formatting of the Disqus system’s built-in solution.