Marvel has just released a press release stating the new dates.
Iron Man 2 on May 7, 2010 (unchanged)
Thor on June 17, 2011 (changed from July 16, 2010, that makes sense)
Spider-Man 4 on May 6, 2011
Captain America on July 22, 2011 (changed from May 6, 2011)
The Avengers on May 4, 2012 (changed from July 15, 2011)
The plan is to have all the movies (except most likely the Sony produced Spider-Man) interwoven together for the release of The Avengers. We will get a Marvel superhero movie each summer from 2010 to 2012.
Start saving your money for popcorn...