This news comes from the Apple Developer news blog, where they say that you can now download even larger apps over the air (i.e. on 3G or LTE, or EDGE if you still use that). We reckon this is to do with the new larger size of many apps, especially ones for iPads, as they now have images that can support the large Retina Screen.
Great apps come in all sizes. If you happen to offer a larger app, we’ve made it even easier for App Store customers to get your app wherever they are. We’ve raised the Wi-Fi download requirement from 20 MB to 50 MB so customers can download apps up to 50 MB in size over their cellular data network.
One thing you should be aware of is that your data limits set by your provider could easily be eaten up quickly if you download too many 50MB files over the air. We also guess that this will include downloading podcasts. Our plan currently allows us to download 500MB, for our iPhone, so we will definitely need to keep an eye on things.