Red Baron was a kind of vibrating app despite being an alleged game. However, massager is the real deal and it's free!
There's a couple of them. I started a thread about the first one. This one doesn't try to pretend it's something else. The icon is subtly... well... http://itunes.apple.com/WebObjects/MZStore.woa/wa/viewSoftware?id=301940380&mt=8
Give it time. There'll be 3rd party "water"proof cases coming out. I think this app was in another thread?