CCSM answer for 11.04
On Ubuntu 11.04, the compizconfig-settings-manager (CCSM) "Put" plugin can be used to set a shortcut for moving the active window to the next output or monitor in a multiple monitor system. In the Bindings tab of the Put module under Window Management, set Ctrl+Alt+z (or other if this does not work) as the shortcut for "Put to Next Output"
If Compiz sees your setup as a single monitor, use "Put Within Viewport" Left/Right instead (forum thread 1588821)
Warning about CCSM on 11.10
CCSM is broken on Ubuntu 11.10, in part because it uses gconf2 backend, in part because it disables Unity plugin due to keybinding conflicts with its own plugins. As a result, the unity launcher disappears.
To get the launcher back, remove the ~/.config/compiz-1 directory. You can obtain a terminal with Ctrl+Alt+F2, or a Nautilus window with Ctrl+N from the desktop (from where you can run /usr/bin/xterm), or Ctrl+Alt+T to launch Terminal. For ccsm config purge, remove .gconf/apps/compiz-1/ and .gconf/apps/compizconfig-1/ and ~/.compiz-1, and then run unity --reset for unity defaults.
Way forward: Using dconf and dconf-tools it may be possible to configure a shortcut by directly modifying the config tree.
Postscript by @DrSAR: The CCSM steps did not work for the OP (even though others have benefited from it). This might be related to the problem of compiz seeing only one monitor.
PS: Use at your own risk since somehow installing and using compizconfig-settings-manager eventually disabled unity for the OP to the point of a seemingly irretrievable launcher and panel.