Although Horace Burrell said CONCACAF are considering a final group of 12 teams, it seems they are actually planning an octagonal, at least according to Radio HRN.
The Radio HRN article is basically the CONCACAF press release, with a different title and subtitle.
CONCACAF clasificaría al próximo Mundial mediante una Octagonal
Durante la reunión del Ejecutivo de la Confederación en Nueva York (participó el hondureño Alfredo Hawitt Banegas), estudiaron crear cuatro cuadrangulares en la fase previa a la Octagonal.
Basically, it seems the final round will be an octagonal, with four groups of four in the previous round.
Did Julio César Núñez, HRN senior sport editor, get inside info from Mr. Banegas? I don't know. I certainly have been unable to get info from Mr. Gulati or Mr. Alvarado, the only members of CONCACAF's Executive Committee kind enough to answer my e-mails.
Also, this post on BigSoccer has a few details about the preliminary round and the first group stage.
All things considered, it looks like this:
1. Preliminary round to get to 32 teams. CONCACAF have 35 teams, but the number could increase to 36 with the Netherlands Antilles dissolving (more on Wikipedia).
2. 8 groups of 4 teams - top two advancing to the next stage.
3. 4 groups of 4 teams - top two advancing to the next stage.
4. One group of 8 teams, with the top three qualifying for the World Cup, while the fourth placed team will go to an intercontinental playoff. Of course, that's if CONCACAF will have 3.5 spots.