I'm wondering how to set the mouse cursor position under X11? Is it possible at all and if, where do I have to look for appropriate functions? X window system, KDE/Gnome/...?
Sounds like you're using X, so what you probably want is XWarpPointer. To give an absolute position on the whole screen, use the Root Window as dest window.
(You can get a quick and dirty list of X functions using ls /usr/share/man/man3/ | grep '^X')