Miserlou - any aerofoil affected by considerable ice accretion will have a totally different Cl-alpha curve compared with the same aerofoil in its uncontaminated state. The alpha value which will trigger stall protection devices will be set up for the uncontaminated aerofoil, hence it has no 'knowledge' of the stall alpha of the contaminated wing. Thus a wing with considerable icing may well stall at an alpha value below the 'normal' stick-shaker value.
Flying as SLF in an ATR72 twice next week - shall keep an eye out for ice...