May be it's an old question, but i think it's important. As we know, BSIM3 spice model does not give a clear definition for 'vdsat', and vdsat is always not equal vgs-vth.
And so, which can be taken as 'criterion' in out circuit design.
Well... vdsat = vgs-vth is a very basic first order expression. There are may factors in your spice model that are used by spectre or any other simulator. it is just to give u a rough hand analysis.
So u could probably look at the spice model and equations for vdsat....