If you want such a PC you have to buy a special mainboard with two Socket 939.

Until now every Socket 939-CPU had only one non-coherent Hypertransport-channel so they couldn't be connected together without special hardware. In Opteron 2xx- and 8xx-series you have three Hypertransport-channels which are partially coherent (Opteron 2xx: one coheren HT-channel, Opteron 8xx: all three channels are coherent).
Since the cores of the Athlon 64 and the Opteron are basically almost the same but partially deactivated (HT-channels for instance) at the Athlon 64, AMD can activate a coherent HT-channel at the Athlon 64, too.

Basically it's the technology of the Opteron which enters "normal" desktop-PCs.

Here is a german newsticker which has a drawing of the principle of the 4x4-socket.
In the [Update:]-section is written that - different from earlier speculations - the 4x4-technology is able to split the controlling of the memory to both CPUs so the CPUs don't have to share memory like all Intel-CPUs.

If anyone has an Airbus full of cash, please deliver to my address so I can buy one of these mainboards!

LOL!