I don't think this made it to any of the documentation:
When the project drawer has key focus, you can press 'space' to activate rename, and return to open the selected file (this should probably be supplemented by cmd-down as in Finder).
To activate rename with the mouse, hold down option while clicking the item (or use the context menu).
The reason why groups cannot be double clicked to activate rename (as some have requested) is that I plan to do it more like Finder in the future, so that double clicking a group will enter that group, and make the project drawer only show the files contained in the group (there'll be a back button of course).
Kind regards Allan