they can be interchanged, in most recipes, but the taste will differ...I have heard bmilk makes cakes extra moist... I only use that when I make my waldorf astoria red velvet cake.....
I know if you use buttermilk in pound cake...it gives it a wonderful moist taste. I have never used milk or buttermilk in a regular cake mix. I have read some comments that it does not taste good...so I am afraid to try it. I guess I need to do it on a practice cake for my family! 
traci
You can substitute milk for buttermilk if you will add some vinegar or lemon juice to it first. Here is the ration: 1tsp. vinegar or lemon juice for every cup of milk.
It is a whole chemical reaction thing, adding acids to bases, etc. It works! I don't know anything about substituting buttermilf for reg. milk though.
my granny has always used milk in her cake mixes instead of water and everybody thinks they're homeade. She's now 80 and still baking up a storm. She uses reg. whole milk and canned milk either one.
That you can do. I'm not sure what effect buttermilk would have on the cake, I'm almost certain it wouldn't turn out as it should. As patricia said buttermilk has a different effect because it's more acidic than regular homogenized milk. So if you substituted buttermilk in place of plain milk you would have to alter the leaveners. I feel like I'm back in science class just thinking about it..lol
The reason you use buttermilk is for it's acidic reaction with the baking soda and/or baking powder in the cake ingredients. Simply using plain milk instead will mean less leavening action since plain milk isn't very acidic; the baking soda won't fizz up and the batter won't rise as expected. But it's very simple to substitute.
As luv2cake noted, for each cup of buttermilk required in the recipe, use 1 cup regular milk + 1 tbsp vinegar. Be sure to let this milk stand for 10 minutes before you add it though, so the vinegar has time to do it's thing to the milk. Once it's sat for 10 minutes, give it one last stir and add it as directed.
You can find more details on this and many other handy substitutions in "The Joy of Cooking".
I'm not sure what effect buttermilk would have on the cake, I'm almost certain it wouldn't turn out as it should.
I use buttermilk in cake mixes when I make the Red Velvet mix from DH. I also use it in chocolate mixes occasionally if I have it on hand and want to use it up. It doesn't have an adverse effect and everyone really loved the Red Velvet when I make it.
I don't notice a diffence in texture. Just adds a little something to the taste.
I was making an Italian Cream cake late one night and it called for buttermilk and baking soda, I used whole milk and baking poweder instead. I know that the two have a chemical reaction that will make the cake rise. It came out just fine.
In the cakes I make everyday, I always use milk instead of water and if I run out I will mix up canned evaporated milk with water and use it. It give the cake a nicer texture and people that say they used to make homemade cakes always compliment me on how good the cakes are. I also attribute that to the special rum mixture that I use in all of my cakes.
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