Like it was said before me: the important thing is that you use CAS in calculations (well, the mathematical correct way is to use EAS, but CAS = EAS in spam cans):
Vs = Vs0 * sqrt(m/m0)
Again, as said before, IAS tends to become lower than CAS with increasing angle of attack. For example, quick glance at an ASI of C172P would reveal stall speed of 33 KIAS. One might think Cessna made significant improvement to aerodynamic characteristics of C172 to allow normal cruise speed combined with very slow stall speed. But a bit more detailed analysis of the performance chapter of POH gives you the real stall speed - 46 KCAS - which is normal for this kind of aircraft.