Unlike FFXI where all jobs are availible for all race, WoW only leaves certain classes for certain jobs. Because of this, all races can do all jobs availible with no problem. Only difference between races will be racial stats and traits. Aside that, being a gnome mage will be almost similar to being a troll mage. Only difference is that a troll mage will have higher hp then a gnome and will have the racial trait "troll regeneration" once racial traits are implemented (and if the trait actaully makes it. just picked the trait they had in WC3) A gnome will most likely have higher intelligence then trolls which will result in higher mp.
As for the rundown on what races can be what, here ya go:
Humans: Warlock, Warrior, Mage, Paladin, Priest, Rogue
Gnomes: Warrior, Rogue, Mage, Warlock
Dwarves: Hunter, Warrior, Paladin, Priest, Rogue
Night Elves: Hunter, Druid, Warrior, Priest, Rogue
Orcs: Hunter, Warlock, Warrior, Shaman, Rogue
Trolls: Hunter, Warrior, Shaman, Rogue, Priest, Mage
Taurens: Hunter, Druid, Warrior, Shaman
Undead: Warrior, Warlock, Mage, Rogue, Priest