This bug has been reported on Ubuntu Launchpad as:
I'm using Xubuntu 9.04, fully updated.
The workspace will change anytime the mouse pointer reaches the edge of the screen. The "Wrap workspaces depending on the actual desktop layout" option is not selected in the Window Manager Tweaks options.
I expected the mouse pointer to stop at the edge of the screen, but it will select the next workspace.
Using fusion-icon, I can switch to KWIN, Compiz, or Metacity, and not have the problem, but changing back to Xfwm4, the problem will reoccur.
Changing "Applications -> Settings -> window Manager; Advanced tab and uncheck "Wrap workspaces when pointer reaches the screen edge" has no effect on this.