Weird, now it seems to happen sometimes and only on some forums, but not on this one.
The trackpad problem also seems to be an issue when scrolling over embedded YouTube videos.
But even with no flash, I've been on forums with really weird CSSs, and that has caused some sketchy trackpad behavior.
Edit: Nope, it's, well, I was just able to reproduce the problem here, now it's fine... it's like 4 out of 5 times it works fine, but it always works fine when I use the rest of the trackpad. Stubborn, hard to describe further.