I don't see why the 2013 World Cup couldn't be in the States. It's the most popular sport in QLD and NSW and they could only get an average attendance of 16k to last year's world cup. In the United States, high schools get that kind of attendance for sporting events, not international world cup matches.
It would not at all be dissimilar to the 1994 FIFA World Cup. There had been professional soccer in the United States from the late 1960s to the early 1980s (including the famous New York Cosmos and their star Pele) but almost all of the players were foreign--at times, American teams would simply import entire foreign clubs to play for them. By the 1980s, the professional league had dried up and the only pro soccer in the US was indoor soccer. The US was awarded the FIFA World Cup on a development platform--the country hadn't even qualified for the World Cup in 40 years, yet FIFA realized the country was a sleeping giant when it came to soccer. Lo and behold, the US qualified for '90 and made the round of 16 before losing by a goal to eventual winners Brazil.
The soccer infrastructure in the USA in 1994 was slim to none. There was no grassroots movement, no television, no professional game, and seemingly no future. However, the World Cup got crowds of 60k+ to every game across the country--setting a record for attendance that hasn't been beaten yet, despite the fact that since 1998 the cup has had 32 teams instead of 24. As a result of the cup, MLS was developed (FIFA made the USSF introduce a pro league as a part of the world cup deal). MLS struggled early on but has become profittable and has some of the best players in the world today.
League would be starting in a similar situation, with one major exception--League is a lot more American-friendly than soccer is. Most of us are bored to tears by soccer, whereas league is perhaps the most similar game to American football there is. In fact, the fact that League is even harder-hitting and faster-moving might make the game BETTER suited to the US than American football.
The USA is huge--over 10x the Aussie population. This means that a much smaller percentage of the population can be aware of the RLWC with similar turnout.
The soccer-specific stadia throughout the country, which seat 20-25k, would be perfect for preliminary round games, and I think that they could definitely sell out every game (if Nigeria-Bulgaria soccer could get 44k in Dallas in 94, then I think that Ireland-Samoa or whatever could get 25k). The final could be held in a large NFL stadium and I think it would have a good chance of selling it out.
The gate returns are going to be better than in the UK or Aus just because of the larger population, so long as the tournament is promoted well enough. If the new NRLUS is launched and successful, RL will already have the coverage in the states to attract people to the cup.
FIFA was willing to take the leap of faith on the USA, and the States are fast turning into one of their most lucrative markets. The RLIF should do the same and welcome a powerful new nation into the fold. The AMNRL is a great comp but it has absolutely no exposure whatsover--there are 8 teams and most of the players are just playing in the off-season from union. However, a world cup in the states could honestly lead to a professional competition that could soon rival the ESL and NRL.