The following units have been confirmed by Garry Gaber (project leader of Force Commander):

AT-AAs (Anti-Aircraft)
TRSDs (Tracked Shield Disabler)
Rebel Attack Tanks
Rebel Hover Tanks and Transports

From several of the screenshots of Force Commander leaked by LucasArts 'Roy the Starkiller' has determined some of the units which look like they will make it into the final game. You can see the pics he has worked this out from here.

Imperial

1. AT-AT
2. AT-ST
3. AT-PT
4. Probe Droid
5. Tank Droid
6. TDSD, looks like a Juggernaught
7. TIE Bomber
8. AT-AA- looks like a very small AT-AT, but with sprawed out legs.
9. Laser Turret- like in Rogue Squadron

Rebel

1. Shield Generator
2. Golan Arms laser cannon
3. Radar Dish laser cannon (like at Hoth)
4. "Heavy 1"- distorted picture looks a bit like a Biker Scout