Mifare Ultralight C cards have 3DES-based security, cloning protection and read-only locking [1]. So I don't see how copying the RSA card to the towel would work unless the RSA conference totally messed up their use of RFID or the author has some super-hacker key extraction technique. Am I missing something?

[1] http://www.nxp.com/documents/data_sheet/MF0ICU2.pdf