Almost every other bit of MP3 software I've come across (Winamp, Sonique, libmad, Napster, Audiogalaxy, even Lame) uses the ".mp3" extension whatever layer audio is used.

I suppose, in fact, a better way of putting that would be to say that all software that understands both layer-2 and layer-3 uses the same filename extension for both. And as, by definition, any layer-3 codec is also a layer-2 codec, that would include any layer-3 capable software -- such as all the ones I listed.

Peter