The Czechs have yet to (and may never depending on how things progress with the Euro) move away from their own currency, the Czech Koruna, or Czech Crown. Prices are usually displayed as, for example, 99,00Kč = 99 Czech Crowns. Some establishments may accept Euros but there is no legal obligation for them to do so and large Euro notes (over €50) are unlikely to be accepted. Also bear in mind that the exchange rate that will be used will definitely not be in your favor.