According to AFC, "The members were informed that 20 teams, who have confirmed their entries for the qualifiers, would be drawn into five groups of four teams each. The top two teams from each group and one third best-placed team from among all the groups would qualify for the Finals."
According to QFA, "On the margin of the AFC executive committee meetings, it was confirmed that the qualification stage draw for the 2015 Asian Cup - Australia will be held on October."
AFC had reserved two match days in September 2012 for a possible preliminary round (1 and 5 Sep), but they won't be used.
These are the match days (2013 and 2014):
MD 1: 6 February 2013
MD 2: 22 March 2013
MD 3: 15 October 2013
MD 4: 15 November 2013
MD 5: 19 November 2013
MD 6: 5 March 2014
Alternative match dates: 11 January 2014, 18 January 2014 and 25 January 2014. Why are they needed? Because MD 3 and 5 collide with the AFC-CONMEBOL World Cup play-off.
Now, about participation. This is the list I've received from AFC (yes, they've finally replied to my e-mails!): Uzbekistan, Qatar, Jordan, Iran, Iraq, China, Bahrain, Syria, UAE, Kuwait, Saudi Arabia, Oman, Thailand, Yemen, Vietnam, Malaysia, Singapore, Indonesia, Lebanon and Hong Kong.
Based on the 2011 Asian Cup performance ranking, the bottom 23 will participate in the AFC Challenge Cup, while the top 23 will be involved in the main qualifiers. Obviously, the top 3 (Japan, Australia and Korea Republic) have direct entry and that leaves 20 teams to participate in the qualifiers.
Korea DPR are a special case. Although they are in the top 23, the AFC Executive Committee has allowed them to play in the AFC Challenge Cup because of difficulties in hosting home-and-away games in Korea DPR. If the North Koreans would have confirmed their participation in the main qualifiers, there would have been a preliminary round involving Lebanon and Hong Kong.
Now, there's only one problem. Based on 2011 Asian Cup performance, India are 16th. Why won't they participate in the main qualifiers? No answer from AFC (yet) regarding this.
These will be the pots for the October draw:
Pot 1: Uzbekistan, Qatar, Jordan, Iran, Iraq
Pot 2: China PR, Bahrain, Syria, United Arab Emirates, Kuwait
Pot 3: Saudi Arabia, Oman, Thailand, Yemen, Vietnam
Pot 4: Malaysia, Singapore, Indonesia, Lebanon and Hong Kong.
Thailand should be below Yemen, but this doesn't affect the seeding.
Group of death: Iran, China PR, Saudi Arabia, Lebanon.