Personal comment: Seems a little impractical to me, especially when you have the alternative of not using RFID at all staring you in the face.
You only have that option if you don't ride the underground (or any public transportation, for that matter) in larger cities. At least, that's how it works in Lisbon, the capital of my country. Still, I think it's impratical, as most people just have the card in their wallets, and this'd force them to bring the card out. I'm pretty sure this won't have much success, unless people start being seriously "ellectronically mugged" every day.