Via The Original Winger. Could this be the first step to promotion/relegation?
USL carries three levels, an amatuer Professional Development League grouped into several conferences, and two professional level leagues, USL-2 and USL-1. USL-1 has a couple of teams, Montreal and Puerto Rico, who could compete in MLS right now and could make a decent run for a playoff spot. Portland and Vancover will join MLS in 2011 and could compete right now.
If MLS actually buys the USL, I think this would be a good step for U.S. professional soccer. Even leaving aside the issue of promotion/relegation, a unified league structure will allow for the movement of players in a much easier manner and more importantly, allow, I think, for better development of young American talent, not only for the league but also for the national team.
Keep an eye on that development as it will be important for American soccer.