The front 'Z' echoes Zidane's initial (Zidane / Zizou), it's also a reference to Zorro's Z, a reflection on how footballers are perceived as heroes.
The reverse of the coaster spells out ZUT, a french word that might be used to replace a swear word of any sort, without being perceived as improper or inappropriate use of language. ZUT incarnates both a polite version to the fans reactions when Zidane head-butted Materazzi and Materazzi's behaviour during the game, leading up to the incident. It's also a 'polite' word french people say frequently when they spill their drink so seemed appropriate for a coaster.