It appears that the rocket launcher, at least in the Public Beta, has lost its ability to lock on to vehicles, and has returned to its Halo 1 configuration in that respect.

What does have the ability to lock is the new Missile Pod, another heavy weapon.

Like the portable turret, you can't zoom, drive or ride in vehicles, run, or crouch while wielding it.

NOTE: Information from the beta version subject to change!