Looks like the release date change for The Avengers isn't because Marvel was worried about The Avengers going up against Harry Potter and the Deathly Hallows Part II in the summer of 2011. AintitCoolNews reports that a Marvel source has told them that the reason for the delay was not only financial (Marvel will have to pay "significant" money to Mickey Rourke for Iron Man 2 and Samuel L. Jackson for nine Marvel movies), but also because it gives Jon Favreau the best chance to direct The Avengers. The source claims that Favreau would not have had time to direct The Avengers after finishing the "accelerated schedule" of Iron Man 2 unless The Avengers had been delayed for another year, and Marvel wants to give Favreau the best chance to be available.
AICN's source wasn't done with The Avengers news. The source also confirms that Alexander Skarsgård is Kenneth Branagh's choice to play Thor, and that Josh Hartnett will likely play Thor's adversary Loki. Stellan Skarsgård is a strong possibility to play Odin, Thor's father.
Fox is indeed rebooting the Fantastic Four franchise, the source confirms, as well as Daredevil, because if Fox doesn't make a new movie based on those properties every few years, the rights revert back to Marvel.