According to Frankie in Bungie's latest Weekly Update, the artificial intelligence in current builds of Halo 3 seems to be quite a step up from the previous games.

The progression of talent in Bungie's AI drivers is well-documented; marines, for instance, could only handle Ghosts in Halo 1, often with disastrous results. Vehicles were thrown open to all kinds of units in Halo 2, as marines would drive Warthogs and Scorpions if given the chance, Grunts would take turns in Ghosts if opportunity arose. Having a passenger (preferably with a sniper weapon or a rocket launcher) as well as someone manning the chaingun in the Hog really gave the vehicle quite a presence.

Gunners in Halo 2 were often not quite as effective as they might have been. There was no real way to control what they were firing at, except to get closer to the intended target. Often they'd ignore clearly visible enemies because they were "out of range"-- even though the weapon itself would have been quite effective.

And Frankie says that's getting better in Halo 3:

In previous Halo games, the gunners were smart and would pick off targets in a logical fashion, but this time around, and it could be my imagination, the gunners are more precise and easier to influence. You can target individual bad guys by steering the Hog and even “convince” your gunner to pick a more relevant target and have him stop trying to blast a lone grunt hiding behind a rock when a bigger menace approaches.

[snip]

The AI still has ages of tuning left to go, and the bad news is that if your AI gunner is too good, he’ll be dumbed-down to make the game more balanced. That said, the scale and ferocity of the current encounters (read: more bad guys than ever before) mean that you were going to need more capable AI anyway.