USA Today has an article suggesting that the imposition of the salary cap in the NHL before the 2005-2006 season has improved competitive balance. Stacey Brook argues "not so fast my friend." Stacey has done the heavy lifting and argues that competitive balance has been improving over the past 30+ years, most likely to a larger pool of people with the skills to play hockey at the NHL level.
One explanation for the lack of effect from a salary cap has to do with Rottenberg's Invariance hypothesis. According to this hypothesis, players tend to play for the team that values them the most. A change in property rights regarding who has the final say over where a player plays does not alter team valuations (the value of the players' marginal contribution to his team, assuming teams maximize profits) and thus does not alter where player play. The salary cap essentially shifts the ability to capture value from players to teams, but does not per-se change players' values.