I have an application with LSUIElement set to 1. It has a built-in editor, so I want the application to appear in Cmd+Tab cycle when the editor is open.
You can set App "Application is agent (UIElement)" to YES in your plist file.
EDIT:
I think there are some hacks to do this. But it's really not the way it's meant to be. Cmd+tab is for getting an application to foreground, but if you don't have a menu bar, it doesn't look like foreground to the user.
I'd rather make a menu bar to access the app.