"The computer behind the display could be used for several tasks, like a server or just for having a backup computer in case that the MacPro goes down."
I was in the market for a MP + 30" ACD for several months when the 27" iMac launched. I happily have the iMac setup as a UPnP media server and development web server with the MP connected and used as my main machine. A simple Command+F2 from the iMac keyboard will alternate between the two machines. I use the wireless keyboard and mouse tucked away in a desk drawer for the iMac (since I rarely need to use the "guts" of the machine) with the wired keyboard and mouse connected to the MP. All in all I love the setup. I just couldn't pass up the iMac at $1699 with the ACD at $1799.