Yeah, I brought this up a while back and nobody really had a good answer.
Is a first-level kobold fighter gonna start with 1d10 or 1/2d10 HD? What about a sorcerer? 1/2d4 is harsh...
I think it's pretty well assumed that a 5th-level kobold sorc has 5d4 HD but is it spelled out anywhere how that first half-HD affects things? Not that I've seen.
According to Savage Species, races that only have one hit die lose that hit die when they take a character class. So a 1st level kobold fighter would have 1d10 hit points (10 hit points since first level characters get max HP).
There'd be a few other little differences too, which have been hashed over before. The kobold in the MM is not a 1st level warrior. He's a 1/2 hit die humanoid without a character class, like it or not.
As others have stated, the half-hit die disappears and is replaced by a full hit die when they take a class. See MM p. 14, under "Creatures With Character classes".