Some people are experiencing a problem when attempting to download podcasts from the iTunes store after upgrading to iOS4 on Apple's mobile devices. They receive an error, which states "podcast name cannot be played on this iPhone."
Resetting the device to factory settings does not appear to help, and indicates the problem is more likely a bug with iOS4 than an issue with the podcast encoding or setting that could be switched to fix the problem.
Furthermore, a few people have noticed that when they download the podcasts through iTunes and copy them to their iOS devices, they work just fine. This supports the idea that this problem is a bug in iOS4, and may need to be addressed by Apple in an update (maybe with the upcoming iOS 4.1 release). Until then, the workaround is to use iTunes on your computer to manage podcasts instead of using the iPhone, iPad, or iPod Touch itself.