To change my input between Canadian and Canadian French, I use Shift+Command+Space. This can be configured in System Preferences> International>Input Menu, then Input Menu shortcuts. By default, in Tiger anyway, it's set to the same as the Spotlight shortcuts, which are used instead, so you do have to change them.
EDIT: Sorry, you can't CONFIGURE the shortcut there, it simply lists them. There should be a Keyboard shortcuts link. Clicking it should bring you to Keyboard shortcuts, where the Input Menu ones can be changed.