Today I was using the wall on the desktop with my Ubuntu 12.04 and I noticed a weird behavour: the edge works onloy the 1st time.
After a bit of debugging here is a workaround:
$ sudo apt-get install gconf-editor
Open gconf-editor, go to apps > compiz-1 > general > screen0 > options
edit the active plugin list so that wall should be down on the list. I have it before scale addon.
Now if you haven't before you have to enable edge flipping and bindings in compiz config settings manager (see image) and voila'...
hope you find it useful