For five years, Apple's iTunes Music Store has been the Internet's most successful music store. But as music publishers have sought a higher share of its proceeds, Apple has threatened to shutter iTunes. The Copyright Royalty Board in Washington, D.C. is expected to rule Thursday on a request by the National Music Publishers' Association to increase royalty rates paid to its members on songs purchased from online music stores like iTunes. The publishers association wants rates raised from 9 cents to 15 cents a track - a 66% hike. Apple (AAPL, Fortune 500) declined to discuss the board's pending decision or its previous threat to shut down iTunes. But it adamantly opposes the publishers' request. In a statement submitted to the board last year, iTunes vice president Eddy Cue said Apple might close its download store rather than raise its 99 cents a song price or absorb the higher royalty costs.