Does anyone have a good recipe for a frosting that is "not too sweet" but I can still use to do simple borders and decorations with on a cake or cupcake?
I've had a couple of people ask me lately for icing that isn't too sweet. I normally use a basic butter cream recipe.

http://www.cooks.com/rec/view/0,1713,144186-252205,00.html

OK let me be clear, I Have Not Tried This Recipe Yet. But I've heard it isn't too sweet. A lot of WWII recipes used this cooked flour method. I need to make a practice batch some day to see how it comes out.

Try any of the meringue based buttercreams. It takes a little while to get the hang of them. My first batch was a bit too runny, next time I'll make sure my butter isn't so soft. They taste awesome and not near as sweet as traditional buttercream. I would use them for cupcakes and fillings but I'm not good enough yet to use them to smooth a cake. It would be great on a traditional rustic style cake that intentionally shows spatula marks. Just search meringue buttercream on google or on CC and you'll come across a ton of recipes.