Chances are you won't really notice the difference if you are just subbing the milk for soy milk. Will you also have to replace the butter though? Unless your recipe uses oil for the fat that is. 
Soy milk isn't horrible, and as an ingredient in a cake it's not going to leave a huge taste, and if they are used to eating soy products then they will not think it's off.
Soy milk is generally sweeter than regular milk so that will be one of the subtle differences. Other than that I would think it would make a big difference.
It's really a taste preference. I personally do not like soy milk in baked goods, I think it has an aftertaste. I use a lot of rice milk since I cannot always use milk. If they are asking for soy milk then they are used to the taste and I say go with it.
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