Zarhym responded to a thread on the official forums started by, of all things, a troll. The poster wrote accusing Blizzard of lying about paid race changes being still in development, pointing out that faction change enabled racial changes, as well. Zarhym calmly explained that "the conversion tables for changing races within one's own faction are completely different from the faction change service." This means that there's a separate development time for it.
He also reminds everyone that "the only thing better than offering a service people want is making sure that service works when people want it." It's pretty much standard operating procedure for Blizzard, a company notorious for pushing back releases in favor of getting things right. It's interesting to note that Blizzard introduced faction changes ahead of race changes, perhaps because the former addresses more player concerns while the latter is merely a customization preference.
There's also a chance that Blizzard will allow the new race-class combos leading up to the Cataclysm, which would be perfect timing for the paid race change service, anyway. I was looking forward to swapping my Blood Elf Death Knight into an Orc, but since the announcement, I've been drooling over the prospect of an Undead Hunter -- a race-class combo I've wanted to play since I first launched the game. I've waited this long, I guess a little more of Soon won't hurt.