Apple's had a long-standing limit on iOS and the App Store: if your download is more than 20MB, you'll have to download over Wi-Fi. With the release of iOS 5.1 today, that has changed — users can now download up to 50MB over 3G (or now LTE networks with the new iPad). Of course, you're still out of luck if the file is bigger than that, but 50MB should cover many apps, podcasts, and pretty much any song out there you could want to download. If you're interested in video, though, you'll still need to hook up to Wi-Fi or your computer.
Apple increases over-the-air download limit for iOS to 50MB
iOS 5.1 now supports over-the-air downloads up to 50MB from the iTunes store, up from the previous 20MB cap.