I made a request for Numidian Cavalry a really long time ago. (Both here and at CDG)
The good news is Kryten is working on it. The bad news is Kryten has to finish other stuff first. I bug him occassionally to do it, but I'm patiently waiting until Christmas before I will even need the unit. Maybe Kryten will see this thread sometime soon
My goal for the unit is 2-fold. First off, I plan to make a mod where every civ gets 2 uus and a Wonder. Carthage gets both Numidian Cavalry and Libyan Mercenaries (Civ3's Numidian Mercenaries are clearly Libyan, and, if it wasn't for the fact that "Libyan Mercenaries" sounds an awful lot like a terrorist group, that would be their name).
My other idea is a bit more complicated. I plan to make massive changes to the Rise of Rome Conquest. Rome will get Velites, Carthage will get Baleric Slingers while still keeping their "Libyan Mercenaries". Now the next part is where Numidians come into play. A "Numidian Horses" resource will be added, which allows anyone who has access to them to build Numidian Cavalry. These units are probably among the best in the game (for power, speed, and cost). Carthage obviously starts with the resources, but Rome can feel free to capture them and "recruit" Numidian allies to help defeat Carthage.
Numidian Cavalry was Hannibal's favorite weapon in the second Punic War. They were far superior to the Roman's cavalry which was, to put it midly, very, very bad

. Their unique fighting style was simple. They would ride to the enemy, getting as close as possible. When they get close, they release a shower of Javelins, and quickly turn around a run away before they can be chased. Then, they regroup and begin another attack, hitting with the same manuver.
EDIT: Xen, do you have any info about them?
I know what you say, but, like it or not, Carthage had its military strenght because of its Mercenaries. They could allow it to have the manpower somewhat close to Rome, and fight almost as effectively.