Yesterday I switched from Debian Squeeze to Fedora 15 (XFCE-spin).
The first thing i recognized was that the 3rd mouse-button is not emulated anymore, since I'm using the Logitech Marble Mouse (a trackball with 2 main-buttons) I really need to have some 3rd mouse-button emulated.
A quick search through the web gave me this solution:
Create a file called "99-mouse-midbuttonemulation.conf" in directory "/etc/X11/xorg.conf.d" with the following content:
- Section "InputClass"
- Identifier "middle button emulation class" MatchIsPointer "on" Option "Emulate3Buttons" "on" EndSection
Afterwords you just need to restart Xorg.