I have three users set up on my system (Debian testing / x64 / Xfce 4.8). For two of them scrolling with the mousewheel works perfectly, but for one user it doesn't work properly in Xfce. The mouse-wheel scrolling will work, but only when the mouse is over a scrollbar (horizontal or vertical) (e.g. in Thunar or when over the workspace-switching applet). This method doesn't work at all in iceweasel.
I've tried deleting .gconf* and .config/xfce* but I still can't see why the mouse doesn't scroll. The home partition has survived several linux distro installs, so there may be odd files knocking around, but don't want to lose all settings etc that do work.
Keyboard - Cherry Cymotion Master Linux (Num lock on/off makes no difference)
Mouse - Logitech MX518
Both autoconfigured at startup (attempts to manually configure by creating an xorg config file actually fails with "Number of created screens does not match number of detected devices. Configuration failed")