can not change the number of workspaces

The effect James described is actually an intended effect of the wnck-applet detecting compiz to be running.

For the meantime you can change you workspace layout using any of the following three methods:

1.) gconf-editor
  start gconf-editor via a gnome-terminal or via Alt-F2
  in the tree-structure on the left side of the newly opened window locate...
    /apps/compiz/general/screen0/options/
  on the right side - in the scrollable view - locate entries...
    vsize
    hsize
  change them to your liking by double-clicking the respective entry and alter the value

2.) gconftool-2 (as an example we set up a 4:2 workspace layout)
  start a gnome-terminal
  enter the commands...
    gconftool-2 -s -t int /apps/compiz/general/screen0/options/vsize 4
    gconftool-2 -s -t int /apps/compiz/general/screen0/options/hsize 2

3.) ccsm/compiz-config-settings-manager (not installed by default)
  start ccsm via a gnome-terminal or via Alt-F2
  go to the 'General' section (click the button at the top of the scrollable view on the right)
  go to the 'Desktop Size' tab
  set horizontal and vertial size to your liking
  _leave_ number of desktops at 1

I extended the wnck-applet (its properties dialog) to further increase the compiz-awareness/integration. The fix should show up in an updated package after the beta-freeze.

You will be able to change the viewports/workspace-layout via the properties dialog of the wnck-applet. That very dialog is now able to adapt itself to either compiz or metacity.

gnome-panel (1:2.20.0.1-0ubuntu5) gutsy; urgency=low

  * extended wnck-applet to be really aware of compiz running and
    adapt its properties dialog accordingly so people can directly
    change columns/rows (hsize/vsize) like they are used to from
    metacity, this fixes (LP: #141210)

 -- Mirco Mueller <email address hidden> Wed, 03 Oct 2007 15:28:06 +0200