This is actually possible to do, although perhaps not as straightforward as many people would like it to be. I was able to just do this on a UNR install by following these steps:
1. right click on the indicator applet (the icon that looks like an envelope)
2. deselect "Lock to panel"
3. right-click on it again and select 'Move'
4. move it over a bit to the left, thus leaving an open blank area to the right of it
5. right click in the blank area and select 'add to panel'
I'd like to make this a wishlist item for now, but it could perhaps be converted to a question as well if it is decided not to pursue this as something that could be improved