Return and enter aren't the same function, and aren't interchangeable in OF, at least as it comes out of the box. Which one are you using? You've given two different answers in your two posts...
As of 1.5, the behavior of the Enter key has changed somewhat (from the release notes archive):
Behavior of Enter and Escape Keys: We are trying to move away from the use of the Escape key to confirm changes, as most applications use it to cancel changes. So by default, the Enter key now switches between row selection and text editing modes, rather than moving to the note area. You can still move in and out of the note area by pressing Command-Apostrophe. You can also still use the General preferences to customize the behaviors of the Escape and Return keys in Quick Entry.
With a completely out of the box setup, the return key will get you a new action in an empty Inbox regardless of whether the focus is in the sidebar or the outline. The enter key will get you a beep in that same situation.