April 15th is the last day you can file your income taxes in the United States. As a result, Mafia Wars will be having a special Tax Evasion Event to commemorate this day. Players will receive double mastery in all cities in addition to a special item, The Backdoor (21 attack, 53 defense). It looks like tomorrow is your big day to finish up Bangkok if you are still lagging behind.
Given the frequency of these double mastery days, this leads me to believe that Mafia Wars Las Vegas must be around the corner. Double mastery days help players fly through content - there would be no reason to hasten the completion of content unless the developers had something ready to tack on! Maybe we'll see some announcements about that soon enough!