I am making a wedding cake and the bride wants ribbon around the bottom of each tier. The cake is going to be covered in buttercream icing. How can I cover it and not have the ribbon get wet or greasy?

I this can help you they talk about attach ribbon on fondant cake and bc cake

http://www.cakecentral.com/ribon-ftopicp-6327611.html#6327611

Easy peasy.Line the back of it with clear contact paper. Trim the excess off. Start with a chilled cake, so your ribbon and fingers dont mar and ding the icing. Gently prss ribbon into place. A little smoothin with your fingers, and the heat from your fingers will help adhere it. When you get to the end, pull back a little, trim with small sharp scissors and put a dab of BC to hold the end down. I did that with the little G monogram cake and the red and white and black ribbon cake in my pics. Works like a charm everytime.

The stiffness of the contact paper on the back helps the ribbon sit nice and flat and doesn't bend in and sag anywhere.