A great couple of posts by Nathaniel Grow at Fangraphs about MLB's luxury tax and its erosive consequences for player salary. Salaries as a percentage of league revenue have been in dramatic decline since the inception of the luxury tax in 1996, particularly since peaking in 2003:
Overall, however, it appears that the luxury tax threshold has effectively become a de facto salary cap for many of MLB’s larger market teams, and thus represents an important contributing factor to the players’ declining share of MLB’s overall league revenues.