According to the Washington Post, we have been paying some of these guys to join forces with the US and Iraq Government. Some are tribe leaders who may have fought against coalition forces. Hopefully this does not bite us in the azz.
http://www.washingtonpost.com/wp-dyn/content/article/2007/08/03/AR2007080302322_pf.html
It makes sense to question the reliability of these tribes and militias coming in from the cold, but it also makes sense that they would do so. After seeing the US military close up, and Bush's implacable determination, they may just figure that the game is up. Better to stifle AlQaeda, establish an uneasy peace long enough for the Americans to (largely) go home, then see what differences need settling, without us OR Alqaeda in the mix.