As I do not get replies to my forum post , I'll try asking this question here.
Up until recently, I used to get the zoom slider on my Microsoft Natural Ergonomic Keyboard 4000 working by following the instructions in a Gentoo Wiki page . In short, by applying a small patch  to my custom compiled (ZEN-)kernel the keycodes of the slider are changed to values below 255, enabling the X server to process those keycodes.
Unfortunately, after a recent kernel update I get different keycodes, as if the patch isn't applied at all, although I can confirm that include/linux/input.h is still changed. My keycodes now read:
$ showkey keycode 418 press keycode 418 release keycode 419 press keycode 419 release
I flipped through latest kernel changes of 2.6.38.x  and 2.6.39.x  (that I currently use) but was not able to find a change that might have influenced the (translation of these) keycodes.
Anyone here that has an idea what might cause this issue?
Kind regards, Marcel
 https://bbs.archlinux.org/viewtopic.php?id=125259  http://en.gentoo-wiki.com/wiki/Microsoft_Natural_Ergonomic_Keyboard_4000  http://en.gentoo-wiki.com/w/images/1/17/Kernel-2.6.34-msnek.patch  http://git.kernel.org/?p=linux/kernel/git/stable/linux-2.6.38.y.git;a=shortl...  http://git.kernel.org/?p=linux/kernel/git/stable/linux-2.6.39.y.git;a=shortl...