Looking for a more up-to-date solution for Ubuntu 7.10? See Toggle Desktop Effects with Compiz-Switch.
An excellent feature of Beryl was the tray icon that allowed users to easily switch window managers and decorators. A similar icon for Compiz Fusion is now available but is not included in most repositories. A deb package of fusion-icon for Ubuntu Feisty can be downloaded from here. I have been using it since I installed Compiz Fusion and it works great. I use it to turn off Compiz when I want to play a game and start Compiz when I log in.
Once you install the package you can start fusion-icon from Applications>System Tools>Compiz Fusion Icon.