Turns out the sudden appearance of the "South Central" content for MCLA wasn't actually meant to go live on Live. That means, according to Microsoft, that you should delete it, should you already have it.
Why? "Achievements won't register and could cause you problems with your Midnight Club: LA game saves" according to the Community Manager for Xbox EMEA. That and it looks like some of things given away for free weren't meant to be. Oops!