Somewhat random fact: comment solutions like IntenseDebate and Disqus incorporate Quantcast into their js, so you are indirectly getting counted by such services if you use IntenseDebate, etc. I searched for an out-clause, but found none.
I originally took the Quantcast code off my site b/c it slowed things down and despite having fairly accurate counts, it had skewed demographics. I didn't want to encourage them, so I took off the script. However, my use of IntenseDebate meant that I was still using it.
I don't think IntenseDebate is doing anything wrong, but I would like an opt-out.