The only problem I see with the Plex and Boxee solutions, is lack of an interface to the itunes store. I love the easy access to the itunes store that you get from the ATV software. Boxee does not have this capability. The means I'd be stuck with actually using Itunes on the Mac Mini which is a no go.
Now, if I could run a Mac Mini like an ATV, as well as like a computer with Boxee or Plex...I would be sold.
Ofcourse, Apple at the moment probably does not want its own products competing against each other. Which is why we won't see it.
Why not clone the HDD in the ATV and then port that over the MINI?
I mean an ATV has the same insides and guts of a MINI without the actual OS interface. In fact i know the OS interface is there running in the Background, because when i launch boxee on my ATV, i can see the window in the background with the minimize and maximize and close buttons.
But in theory if you grab an ATV OEM HDD, you can find those on Ebay for cheap, copy it and clone it to the mini, i dont see why it wont boot into the ATV environment, and you can use boxee/itunes all togheter. Only issue you might have is with the wireless part.