Pandora recently changed their APIs for their service resulting in some significant downtime for unofficial Windows Phone apps. Radio Controlled is right around the corner and now MetroRadio (both free and Pro) are back on the Windows Phone Marketplace in full working condition.
While many will ding Pandora for doing this on purpose to block unofficial apps, the truth is more complicated than that. Windows Phone Central member Jiffy Pop received a detailed letter from Pandora discussing the ins and outs of the API changes. Here's a sample:
"We know that our recent release has broken most unsupported Pandora clients, and we do apologize for the surprise and inconvenience that undoubtedly caused to your listening experience.