Roel Verdult, an MSc. student from the Raboud University of Nijmegen, used an RFID [Wikipedia] 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).
There's a video of the relay attack available. [ru.nl] 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.
A PDF Link [rtl.nl] to a detailed write up is available.