From the "Data Mine" it seems Araby is not included... which is understandable... minor faction, with little recognition, similar to Cathay and Nippon. But it would be really awesome if CA could made a DLC for them and give them the significance that Games Workshop couldn't be bothered to do.
Lore-wise... they are like, pirate free-cities, where important people like Nagash, Nefarata, the tomb Kings (Settra), etc etc.... all eventually go to for some fighting or to find important magic lore books and all that... a place where Orcs, Lizardmen I think? also fight around in that area.
Then you have those greedy pigs, Brettonia launching crusades once their peasants have run out of money! being able to change the course of Warhammer history is something I would like to do, with Araby's vast and varied unit selection! being able to repel a crusade, then fight off the orcs/lizardmen and possibly lead an counter-invasion of Brettonia or The Empire.
Seeing as they are kind of based on Arabian, Persian and Ottoman lore, they should be a decent addition to the game, a defensive force with decent naval capabilities (if navy is ever introduced in Warhammer Total War) that actually builds, trades and grows rather than just roams around in massive hordes, tearing everything up.
Their lore, units, etc... can be seen here: http://warhammerfantasy.wikia.com/wiki/Araby
But imagine that! Araby is just slightly south of the Old World... they are practically next door! plus their are even more Orc, Vampire and Tomb King factions in that area.
Edit: Oh... and forgot to mention... THEY HAVE FLYING CARPETS! http://warhammerfantasy.wikia.com/wiki/Arabian_Flying_Carpets