You can, however, work around the issue by remapping Cmd+Return to Ctrl+U using Takayama Fumihiko’s KeyRemap4MacBook (which, despite its name, will work on any Mac running OS X 10.4 upwards). Excel does not offer a direct way to customize this: it is neither a menu command (which would be available to OS X’s system wide shortcuts), nor is it a shortcut configurable via Office’s dedicated Tools → Customize Keyboard… (approximate translation, I’m on a German system) dialog – as you can easily test by calling the dialog and pressing Ctrl+U in the Add Shortcut box (no assigned shortcut will be shown).
