I think in the end (based on what I saw on the World Cup message thread), association football will become a lot more popular in the USA. I cite the following reasons:
1. The recent class-action lawsuits over head concussions and the illegal use of painkillers to keep what should be injured players on the field has scared a lot of mothers from allowing their sons to start participating in American football (participation in Pee-Wee and Little League football has fallen off a lot in recent years). And that could start the decline of American football.
2. Baseball is losing a lot of its allure because the game is too slow by American standards. A nine-inning game that used to take about 2.5 hours to complete now can stretch to nearly four hours!
3. Hockey is way too niche a sport to be popular in the USA. The fighting common in the sport doesn't help things, either.
4. Basketball is still popular, but poor quality of new American players coming into the league (no thanks to the AAU circuit and only one season of college basketball for many college players) will eventually start to hurt to sport. With the ban on hand-checking since 2004, foreign players who grew up playing basketball in European-style sports academies are now a lot more viable in the NBA with the right coaching (read: the San Antonio Spurs). In short, we could see basketball become a increasingly populated with non-American players, and that could hurt its popularity among core fans.
5. Young people are starting to gravitate towards non-traditional sports beyond baseball, basketball, American football and hockey like lacrosse and the extreme sports movement. Association football as such is starting to become more popular among Americans, especially when young people are quite interested in the EA Sports
FIFA 2xxx series of video games (the most popular video game sports franchise in the world right now).
6. The increasing economic/political power of the Latino community--most of whom have parents who grew up loving association football--means association football could really take off in popularity over the next 20 years. You can see it in the increasingly-heated rivalry between the Mexican and US national teams during the CONCACAF qualifying rounds.
7. NBC Sports' three-year, US$250 million contract for the English Premier League means a lot more Americans are exposed to the perhaps the most popular association football league in the world. And that has elevated public awareness of the sport at its highest level in the USA. Interestingly, the very early start times for Premier League games (based on US time zones) has actually helped its popularity, since it generally does not interfere with the start times for the majority of American football games at the college and NFL level.
In short, these changes--plus the fact you can complete a game in around two hours on average--will make association football a sport that will make it more popular in the USA over the next 20 years.