The correct pronunication is “John Wookuh-shev-eetch”, according to my decoding of the IPA notation [ˈjan wukaˈɕɛvʲitʂ] from Wikipedia (http://en.wikipedia.org/wiki/Jan_%C5%81ukasiewicz) using Wikipedia’s help page http://en.wikipedia.org/wiki/Help:IPA_for_Polish.