I'll take a honest stab at answering the question.

Q: When is America going to take soccer seriously?
A: We already have, in the 1920's.

This website has the best history of US soccer on the web. Check it out if you haven't already.

An Overview of American Soccer History

A few excerpts from the above:

quote:
During the mid 1920's, the crowds for games were large, with 10,000 a not uncommon attendance figure. This was on a par with the NFL for much of the 1920's.
quote:
By the mid 1920's, the ASL had reached such a level of prominence that major foreign teams were enticed to perform major tours of the US playing against top ASL and other clubs. Sparta Prague and Vienna Hakoah, an all-Jewish side both toured the US in 1926, to record-breaking crowds. Hakoah's first three games drew 25,000, 30,000 and 36,000 spectators respectively, culminating in the famous May 1 1926 match at the polo grounds in front of 46,000 spectators, a crowd record that stood until 1977 when three consecutive records were set by the Pele-led New York Cosmos.
We have a rich soccer history that has been all but forgotten and buried. The real question is when will we as a nation rediscover the sport.