If there was ever a ripe time for soccer to become something other than an "other sport" in the US, that time is now. MLS seems to have its together. You have a growing fanbase for the sport here, especially among Hispanics.
The problem in the past (well, since the 70s) hasn't been that no one plays the game in the states, but rather that you didn't have the audience for it. So athletes were drawn to other sports that had the audience and were heavily marketed.
It's going to take more than just Beckham. The MLS needs to continue to open up the checkbook for European players, and not just necessarily the stars. The more average European league players who view the MLS as a desirable destination, the better the play and the more likely it will be that international fans will tune in to watch matches.
Soccer won't take over US football, baseball, or basketball in popularity in the US, but the potential is there for the MLS to grow into a rather large and successful franchise. The key will be to be good enough to be a viable option for the average Premier League player to play in during his prime. I think for the forseeable future that the MLS is going to see a hefty portion of its growth come from outside the US. But at some point I suspect that the game will start to draw American eyeballs, especially when American athletes can start drawing salaries on par with their counterparts in the NBA, NFL, and MLB.