Here are some notes that might be helpful if you're experiencing problems getting desktop effects to work in Ubuntu Intrepid Ibex 8.10, after enabling dual monitor or dual head support. In my case, the problem is the Intel driver not supporting a high enough texture size. This is not yet fixed in 8.10, but should be at some point. Here is the link:
The problem is that compiz is checking
GL_MAX_TEXTURE_SIZE and failing if the value is not as high as the Virtual Screen size that's set in
xorg.conf. You can check the value by running:
glxinfo -l | grep GL_MAX_TEXTURE_SIZE
If this is lower than the value in your
/etc/X11/xorg.conf file, compiz won't start! You'll see the error if you try run compiz from the command line.
In the meantime, you can work around this by stacking your monitors on top of each other instead of side by side, thus reducing the virtual resolution to within the required bounds. This works for me as my secondary screen is my TV, so it doesn't really matter what the virtual positioning is.