When scrolling, for example in Assistant or in QtCreator, the scrolling is very slow and seems to be "kinetic", as when I stop scrolling using my MacBook Pro touchpad, the page keeps scrolling for a few more seconds (1-2secs) with a decreasing speed. It's especially annoying at low scrolling speed, because the scrolling always overshoots where I'm trying to scroll to. On my computer, this is especially visible at a scrolling speed of ~2 lines per second in Assistant and alternating scrolling up and down a few times, then watching the scrolling still going back and forth for some time after I stop.
At first I thought it was the scrolling performance that was really bad, but if I switch to another application, while still having the Assistant window visible in the background, if I scroll the Assistant window (even thought it's not focused), the scrolling is perfect : fast, predictable, and it stops as soon as I stop scrolling. It doesn't matter if the app in focus instead of Assistant is a Qt app or not, anything works.
So to reproduce : start Assistant, open any help page, QWidget's for example, notice how annoying the scrolling is by scrolling alternatively up and down a half-dozen times and watch the app trying to keep up. Now press Cmd-Tab to focus another app that still allows you to see the Assistant app's window below, have your mouse pointer be over the Assistant's window without clicking, and try the scrolling test of doing a half-dozen up and down scroll. Notice how much better it is.
I tried a few of the QtDemo demonstration apps, and it also happens in the browser, but not in every page. For example it works fine on http://doc.qt.nokia.com/, but behaves "kinetically" on http://doc.qt.nokia.com/4.7/index.html.
Scrolling behaves fine in the TextEdit and Interview demos.
So either Mac OSX is sending different events when an app is in focus than when it's not, or Qt is enabling a special behaviour when it's in focus.