UNSC UNITS:
Marines: As we know, these are shipped in squads. Due to the lower resolution of some of the pictures, they seem to have heavy armor and helmets. Think ODST-grade. The Grenade ability is activated by tapping a 'secondary attack' button on the controller.
Known upgrades: Rocket launchers, Squad Medic.
Flamethrower Marines: A variant on the Marines, but a separate unit. Imagine the Firebat from StarCraft.
Known upgrades: Improved damage + defense.
Spartans: Misidentified in Gamepro as having Mk I MJOLNIR armor. Shield less, but heal fast. Can be wiped out if not used correctly. Said to carry SMGs.
Known upgrades: Dual Wielding has been discussed.
Warthog: Running into enemies is a secondary attack, ala the Marine's grenades.
Known Upgrades: Chaingun turret, Gauss Gun turret, Passengers.
Scorpion: Heavily armored, difficulty in tracking fast units.
Known Upgrades: Explosive shells, faster turret, Grizzly Variant (Double barreled cannon, flamethrower defense, UNSC Corps faction only.)
Cobra: Rocket artillery, anti-vehicle. Think of the Katyuska Rocket Artillery.
Hornet: Its in. It's an early air unit, possibly a weak one.
Wolverine: Still in. Anti-aircraft, anti-building.
Elephant: A prototype to the Elephant we all know and love, it can train Marines from a stationary position and attack nearby enemies with turrets. Fielded only by the UNSC Fleet faction
Cyclops: A ripoff of the Cyclops unit in Age of Mythology, a 'Gigantic bipedal mech" that levels enemy structures with blows from its arms. Come on, Ensemble, I thought y'all were smarter than this. Deployed by the UNSC Corps faction.
Vulture: Deployed by the UNSC Fleet faction, its a slow gunship that seems to have taken the role of the Sparrowhawk.
Factions: There has been much speculation as to how the factions would be implemented, and whether Halo Wars would need more factions like the Flood. However, it seems that ES is splitting the UNSC into Fleet and Corps factions. There is no word as of the Covenant. In my humble opinion, Fleet should be Army. In addition, I would like to point out that I predicted that they would do this on several occasions.
As for Sentinels... they're in. As Multiplayer only. And they ain't going to be a faction either. No, there will be factories which you can capture and control Sentinels as mercenaries. Make sure that your enemy doesn't take your factory from you though.