It really depends on the type of moisturizer. Lotions are emulsions. Emulsions come in two types, oil-in-water or water-in-oil. Most moisturizers and liquid foundations are oil-in-water types so mixing them together will pose few problems. However, some moisturizers, particularly for very dry or mature skin, are oil-in-water since this is an oilier emulsion. If you mix such a moisturizer into a liquid foundation, it will not be compatable and could cause the foundation separate and become unuseable. Ever notice how some liquid foundations "strip" around your t-zone? This is because the excess oils, cause the oil-in-water emulsion to become water-in-oil, causing it to break down.
I agree with the recommendation that you apply a light moisturzer before applying your foundation. Works like a charm.