Hey Robin
I know what you mean about a top panel...i prefer just a bottom panel too...
However, the main reason i do not mind the top panel in Ubuntu (with unity) is because of the global menu...
It is because of that, that the Chrome and Firefox browsers (i use Chrome myself) gains back the extra web page space...it is as if there was no panel on top at all, because of the incorporation of what would usually be a separate top bar with the top panel of the browser...
If one shuts off the global menu (i believe there is a way to do that...i think with the my unity program) then you would lose that extra space...
And since i don't mind the global menu it works out fine for someone like myself who normally isn't that fond of the top panel...
So, while you can't autohide the top panel (at least not as of yet) i lose no space because of the global menu factor...

And the same is true when applications are open and filling up (maximized to) the screen (like say, Rhythmbox or VLC player for example)...
In fact, though i like and use unity....with the global menu and top panel and using the radiance theme (lighter, more metal-like theme available in settings) like i use, but if you added cairo dock, it would look and work almost like a "mac" clone
