To further clarify rumours and statements made before (and in response to my own post yesterday), AMDZone has the official word from AMD regarding Athlon 64 pin count:
There will be a 754, 939, as well as a 940-pin version of the Athlon 64. The 754-pin Athlon 64 will be for the budget systems. The 940-pin Athlon 64 will be the variety at launch. The extra one pin is required to work on the 6-layer Opteron motherboards. The 939-pin Athlon 64's aren't ready for shipping yet so they will be out a little later on. As for the Opteron situation, that little extra one pin is required for SMP support due to the necessary HyperTransport links.