"Runs Android!" isn't much of a selling point as there aren't any android-specific programs yet. Lets say somebody goes out of their way to port the OS (no small feat, I would think). Then what? You have the latest and greatest mobile phone OS and you can't do anything with it. Ignoring the fact that Pandora isn't a phone, you're still stuck with the fact that Android will only run programs written in JAVA with the special SDK. As cell phone SDKs go it is not bad, but you would lose direct access to the hardware (not to mention OpenGL ES 2.0) compared to the custom Linux that Pandora is getting now.
I wouldn't be shocked if somebody eventually ported Android to Pandora just to prove it can be done, but there is no good reason (at least right now) for anybody to actually want to use it.