If you are from Qt world which has excellent Qt-designer served as a good prototyping tool since its early version, you probably are disappointed about the lack of prototyping tools in Android world.
We already has DroidDraw, but it still has a long way to go, though. I came across a website and it mentions Android GUI Prototyping that adds Android prototyping into your Visio template sets. I am not familiar with Visio and I normally use the primitive toolkit (pen, paper and glue) to do prototyping. I don't know if this Visio stuff works for you.