Yeah, you have to use Wifi to download apps over 20 MB. The podcast restriction only applies to the iTunes app, you can use third-party apps to download larger ones (except in the early days of the App Store, when third party podcast apps were forbidden)
Actually IIRC the whole iTunes music store is also disabled when you're on 3G and not Wifi. (edit: double-checked, yeah it's disabled for me http://db.tt/YT0w0rNQ )
Downloading apps/videos over 20MB on 3G isn't allowed, but you can browse the iTunes music store an buy songs (unless they're over 20MB, which is rare).
It seems to me like a sanity check. If a user accidentally downloaded a few 100MB files, they could easily get over their data cap. This restriction is not device wide, just on the iTunes App/Music store.
Actually IIRC the whole iTunes music store is also disabled when you're on 3G and not Wifi. (edit: double-checked, yeah it's disabled for me http://db.tt/YT0w0rNQ )