No 8800 GT for Previous Mac Pro, so far...

Without official statement, Apple update the Apple Store webpage corresponding to the GeForce 8800 GT.
It is now clearly stated that this card will only be compatible in PCI Express 2.0 enable Mac pro models, in other words the last models.
In other words, it is now clear, without a special GF8800 GT, previous Mac Pro will not be able to benefit from such last generation graphic card.