Try to post a question or an answer in the iPhone and you will know what I talking about
On the 3GS it's so fast that you actually don't notice it happening, which is why I had trouble reproducing it.
I've noticed this behavior myself (3G firmware 3.0)
When I try to write an answer it will start jumping all around the screen and it can be pretty infuriating, one of the biggest reasons why I don't bother using SO on the iPhone very often.
[Obligatory reference for a strong API for iPhone app development among other reasons].