How to play FLAC in iTunes?
Due to the unique features of iTunes, only Apple can add support for FLAC.
And why wouldn't they?
FLAC usage is accelerating, many worldwide bands like Pearl Jam, The Beatles, Phish, Dave Matthews Band, Metallica - the same hip, influential people whose fans Apple courts - are already distributing music in FLAC format, and thousands of users are clamoring for it in the iTunes forums.
Through tremendous effort by various developers, goes as far as possible in allowing some playback capability via QuickTime. But proper iTunes support - tag handling, no import delays, etc. - is not possible without Apple support.
Make your voice heard! Fill out the iTunes feedback form (politely!) and let them know.