Apparently, there is a certain package which needs to be installed, for those keys to become functional. I don’t remember the package name, but you can do a text search for “function keys” in package description, within the software repository.
These laptop should all be covered by the kernel. The module asus_laptop takes care of it. The keys on mine stopped working for a while (first KDE4 releases), but I didn’t bother about them too much. Now (see my signature) they are working. Though the extra keys above the keyboard don’t.
You could try to run sax2 and pick the Asus laptop keyboard, and pick that one in KDE as well.
I have a similar problem with my Thinkpad. All the function keys worked under Gnome but under KDE only the brightness and volume keys work. The suspend to ram / disk, screen lock, eject from dock, etc don’t work.