I don't know what they're aiming for but once it's official it'll show up at the bottom of the RAGE forum.
However unless id has specifically said they're aiming at a Teen rating it seems more likely that this will be rated Mature.
They said when they announced the game that they were targeting a Teen rating. The most recent trailer shows a bit of player instigated splatter though, which is usually a no-no for T rated games.
It seems like you can paint the walls red with blood and still get a T if shooting enemies only generates puffs of dust, but if blood splatters when the player shoots someone, it's an automatic M.
I think the ESRB rates more for overall tone though. I guess we'll find out closer to release.