I have today successfully remapped the return key of my old PowerBook G4 to the enter key beside the right command (apple) key after my return refused to operate and I wait for the spare keyboard to arrive*.
I assigned the enter key the same code the return key had (the code is displayed after double clicking a key). Then I saved the keyboard layouts in ~/Library/Keyboard Layouts after logging out and back in, it was selectable from the list in the International preference pane.
Additionally, somewhere in the documentation says that Leopard seeks for changes in the keyboard layouts folder in order to Include the new layout, so after exiting a file, you are advised to put it out of the folder and back again for the system to include on the list.
* speaking of replacing a keyboard, I received a rather rude answer from an authorized service and I would like to file a complaint with Apple. Does somebody know which could be the best way to do it? I couldn't find any suitable item at apple.com