Can I substitute Soy Milk when a recipe for cooking (not baking) calls for regular (or 2%) milk
You can almost always substitute soy milk for regular 2% milk in baking. Soy has almost as much protein as regular milk and slightly less fat that 2% milk.

You'll probably not notice any difference. I've used it in baking and had good results.

Here's the breakdown (according to the Silk Brand Vanilla Soy Milk from my Fridge):

Soy Milk Stats (per cup):
Protein: 6g, Total fat: 3.5g, Total Carbs: 11g

2% Milk Stats (per cup):
Protein: 8g, Total fat: 5g, Carbs: 12g