Comment 8 for bug 810653

tuxinvader seems to has found the reason for this problem (see Bug #527890). Seems to be a kernel bug after all; Will copy the relevant part of the text here and then try to find out how to reassign this bug to the kernel:

Hi all,

I was asked to be help some one get vertical scroll working on their HP,
so I've just blown the dust of mine and installed Natty. Vertical Scroll
still broken by default :-(

The problem with scrolling is Ubuntu specific. Kernel.org kernels should
just work.

Ubuntu patches add support for a Dell Precision 4500. There is a lot of
additional code in there for making the Dell work, but it uses a similar
touchpad to the HP (same vendor/product IDs) and that is what breaks
vertical scrolling on the HP.