What would be a good substitute for coconut milk in curry sauces?
Half and half or heavy cream is a pretty good substitute. It has similar fat/water suspension, reduces and thickens similarly, and emulsifies similarly. You lose the coconut flavor, but it cooks similarly and you keep the texture.
You cannot substitute coconut cream, because it is too rich, which changes the texture of the sauce greatly, and doesn't do as good a job of getting flavors out of spices.
Incidentally, I highly suggest you pick up a box of instant coconut milk powder. It's not as good as the canned stuff (needs whisking), but because it's nonperishable you don't have to use the whole container at a time. This helps avoid the "oh crap, used the last can!" moments you get with canned coconut milk.
I don't think there can be a substitute! Cream is relatively flavourless and if the reason for the substitution is avoiding saturated fat, cream would be even worse (than coconut milk.)
@Doug: Generally the reason you substitute is hitting the moment where you're like "oh crap, forgot to get some at the store." It does come out somewhat bland if you use cream, but if it's all you have on hand, at least the sauce will get the right consistency, and you'll still get the flavors from the spices.