I was playing a custom game against a friend, and he got in a banshee. I grabbed a rocket launcher, waited for the little diamond reticle to appear, and then fired. The rocket went straight. So I fired the second rocket and it worked.
I also tried this with the rocket hog turret, and it doesnt lock on either, despite the little diamond reticle showing up. Is this some sort of glitch or broken mechanic? It would also be nice if it had a little sound indicator like it did from halos 2-reach. I noticed that the dominion turrets make a sound for lock-ons, but neither the rocket launcher nor the rocket hog turret does.