Roel Verdult, an MSc. student from the Raboud University of Nijmegen, used an RFID tag emulator to perform a successful practical relay attack on the single-use OV Chipkaart (the Dutch RFID public transportation card), that uses MIFARE Ultralights (no crypto).PDF Link
There's a video of the relay attack available. The video speaks for itself.
Roel used a homemade tag emulator that was modeled after Kfir and Wool's "ghost and leech", to perform a simple relay attack. However, anyone can perform the same attack using the RFID Guardian, whose HW/SW is freely available.
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.