I got this quote of Colin Strause (Director of AvP2) from AvPgalaxy,
"You want the hard facts? The trailer is being reworked to include footage from the latest scenes that have been filmed. And the scenes were not shot because the studio didn't like the last cut of the film, as I've said in other threads, they were scenes that the filmmakers had always wanted to be do, but due to time and money constraints during principle photography (which was only 3 months) they didn't get done."
In a nutshell, the reason why they haven't got the trailer out is because they are shooting some additonal scenes so they can add them into the trailer as well.