I finally went to Hey Cupcake yesterday and it was everything I wanted it to be. It is a shiny Airstream Trailer converted into a cupcake stand--topped with a revolving cupcake (pink icing with sprinkles). Hey Cupcake should definitely be on your "List of Things to do in Austin."
The cupcakes were pretty fantastic too. I chose 24 Carrot (carrot cake with cream cheese icing), but running a close 2nd (I'll have that next time) was the Michael Jackson (chocolate cake with cream cheese icing).
I'm ready for another. Maybe I'll see you there.
(It's on South Congress, not far from Guero's--queso and cupcakes--watchout!)