Hey here is some new information about halo 3 * brutes are still there..

Not really a question but anyway, on on of the trailers it shows the Chief, throwing a grenade at his feet and a shield suddenly surrounding him, so maybe that's one?

I have searched around heaps of places but I have never seen or heard of a human flying vehicle or a brute ghost.

I think there is a human version of the Banshie. Its called the short sword. It's a smaller version of the ship you fly away on in halo 1

Yes Brutes are still in, however:
The new grenade is simply called the "Brute grenade" but for a more precise name it is a "spike grenade" which has the similar effect to the plasma grenade as it sticks to enemies and vehicles and also walls, ceilings, etc and it has a 2-3 second fuse making it a useful ambush weapon like a fast mine.There are new weapons including a new variation of a pistol for the brutes called the "Spiker" it has a 60 round clip and has 2 sharp dagger like appendages from the bottom giving it a resemblance to a smaller scaled brute shot.
Another weapon that is new to Halo3 but not to the halo universe is the MA5C assault Rifle, similar to the old assault rifle in Halo CE, this weapon only has a 32 round clip and is your basic starting weapon for multilayer games on matchmaking.
There is a devastating new weapon called the "Spartan Laser" this weapon requires a 3 second charge up time but when fired it unleashes 5 beams of red laser which kills even a Warthog in one shot.
Also there is a new addition called "support" weapons, there are 3 support weapons, The Missile pod(an 8 round lock on to vehicles missile launcher)
Chain guns(these are like the human turrets off a Warthog but you can shoot them when they are on the stand in which it has infinite ammo or you can take them off the stand and it has 200 rounds but it kills anything in a matter of seconds.)
And finally a Plasma cannon(these are like the Chain gun but these are the stationary turret that you see grunts manning on Halo 2)however the ammo capacity of these has not been found.
Swords have been rumored to be able to be duel wielded in campaign but this has not been proved and in multiplayer games the sword no longer has infinite ammo it has 100 power and depletes it's capacity by 10 for every kill made with it.
The Banshee now has the ability to fire the Fuel rod cannon on multiplayer games however it is slow and is hard to aim. However the plasma canons are still as powerful and have a higher rate of fire.
The Wraith can now use it's on board plasma cannons that the AI enemies in campaign could exploit against you in Halo Ce and Halo 2 but we could not, however it is now in a gunners position much like the stationary plasma cannons in the previous games, the rate of fire is like a Grunt turret (seen being carried by the green armored Grunt from halo 2) or the Plasma torrent from a Spectre's main gun plus the Wraiths main cannon has a faster discharge and a slightly faster rate of fire, also for the Wraith, there are positions on the sides of the tank for people to mount up making it a slower version of the Spectre but makes up for it's loss of speed by having a huge Plasma Mortar. Noticeable differences:
-The plasma pistol charged shot no longer homes in on targets.
-The Covenant carbine is quickly becoming one of the most powerful weapons in the game because depending on how fast you can pull the trigger it takes 3-4 seconds maximum to kill an enemy. The Sniper Rifle is the old model from Halo CE where the scope has night vision when not zoomed in.
The new vehicles are as such:
The "Mongoose", basically a small quadbike with a drivers seat and a risky gunner hanging off the back wielding a weapon from his or her inventory.
A new flying vehicle for the Humans rumored to be a "Shortsword" fighter from Halo CE or some kind of Helicopter.
There is speculation for a Brute version of the Ghost but his has not been seen.

Hope this helps
This information has been taken from the Halo 3 Beta Build

Brutes there yes.
It is called the spike grenade and I think it's more like a cross between the plasma and frag.
Human flying machine is called a hornet similar to the banshee yet you can have people ride on the side (hint. This helps in the level where yuo fight 2 scarabs at once in co-op), the other human vehicle is the mongoose it is NOT twice as strong as the tank it is faster but still slower than the warthog, the brute "ghost" is called the chopper and quite literally eats other vehicles it is slower than the ghost and alot harder to control. This game while fun is relatively quick to finish.

Wow. You are just making stuff up, or whoever told you that is making stuff up. The grenade is called a Spike Grenade and it just sticks and explodes. The human vehicle is called the Hornet. The new Elites vehicle is called the Shade. Also the brutes vehicle is called the brute Chopper.

This human flying thing you speak of is called a hornet but I have no idea what your talking about when you say a brute ghost.

The brute ghost is a chopper, and prowler, and the mongoose is an ATV for two people.The hornet is a flying vehicle for three it's slower than the banshee but it has machine guns and missles

Halo 3 has been released for quite some time. This isn't much of a question but, yes there are new vehicles including a new covenant and etc.