The problem with pushing the Inverter to the FF is that MAP has to make this descision before it is known for sure that the FF will be packed into an OLOGIC component. Even if the FF has an IOB=TRUE property, the OLOGIC pack might fail for connectivity reasons and the FF is packed into a Slice where there is no way to implement the inversion (since there is no inversion mux in that path).
Now that Pack supports the new IOB=FORCE property (pack FF in OLOGIC or error), it is possible to ensure that the FF will not be packed into a Slice. Beginning with ISE version 12.1, MAP pushes the inversion to the FF if an IOB=FORCE property is on the FF.
OLOGIC Component with Inversion Mux in Input Path to FF
Slice Component with No Inversion Mux in Input Path to FF