Scrolling behaviour for apps (in general)
I have also seen the 3 scrolling types you mention below, but it always seemed to me that the 3rd one was more a bug than a feature.

The HTC Desire (which I have too) is not known to have a great digitizer, the part of the screen that handles the finger positions and movements. Furthermore, it's not "multitouch", meaning it cannot handle properly more than 1 finger on the screen, although there are tries to support it but I've never seen it work constantly good. See this page for more proof.

It's true that it would be a nice feature, maybe on other phones with a better touchscreen and a further version of Android with more gestures supported. Why not for example scrolling down with 2 fingers would trigger a super-fast scroll or a "go to bottom" action?