Melanie Rieback, who worked on the RFID Guardian, sez,
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).
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.