If you want to use buttermilk for raising something like scones, a few teaspoons of white vinegar in a glass of milk will do the same thing - you need to leave it stand for say 10 mins, and then it is fine to use - it will react to the bicarb/ levure/ cream of tartar or whatever to give the rise required.
Saves having to buy it and use it all up. Much cheaper.
About.com have some good tips on substitutions worth taking a look at. Happy cooking!