Here is their main intro. Controversy concerning the quality of anonymity is described here. Michael Froomkin has written on legal aspects of digital anonymity. Here is a protocol that I devised that was inspired by early rumors of the Mondex card.
Stories about a break in to Mondex.

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Stuff above was written when Mondex was just a couple of years old. A search today (Sept 2005) yields these notes. 1997 June Overview; 1996 Nov MasterCard to take over Mondex; 1998Australian trial; Perhaps a current official site;