this may be a dumb question, but I'm going to ask anyway...how do you get fabric ribbon to stick to a fondant cake? RI?

Yep. You can also iron a piece of wax paper to the back of the ribbon to keep it from absorbing the grease, then just attach with a dot of icing (RI or Butter Cream).

Yep. You can also iron a piece of wax paper to the back of the ribbon to keep it from absorbing the grease, then just attach with a dot of icing (RI or Butter Cream).

On this subject what about ribbon on a buttercream cake. Will it get greasy?

Yes, it will get greasy. The grease will soak through the ribbon and make it a shade darker. If you are going to apply it directly, buy a shade lighter than you need.