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 see

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.