Shintai,AMD needs other (90nm) fab up and running...I guess you can understand that,can you?
It means they use all they have and balance the product portfolio:90nm F3 for highest speed bins in A64 X2 and Opteron line,90nm F2 for mid range A64 X2 and single core line and 65nm for low-mid range(Bisbanes).
They do need the capacity for the K10 QC parts since the die size is quite large.
K10 dual cores will be much easier to produce and bin higher(even if it costs them some % in higher TDP specs)