Don\'t worry, it\'s just a rumor, hopefully not a true one. I have an American accent (Mid West as Americans I\'ve met claim) and I passed both FCE and CPE.
So don\'t believe in such gossips.
BTW, American English is more expansive, as there are mainly American movies and shows on TV in most (European at least) countries, American music is popular probably all over the world, even in Britain Americanisms permeate to the language. Such an amount can\'t be just ignored.
And cambridge are impartial exams certyfying that you have a good command of English. It means that you\'ve aquired and developed the four skills (speaking, wrting, listening, reading) and as long as you\'re fully comprehensible you pass irrespective of the accent. (it can even be a strong POlish one)
Regards,
KOciamama.