I mostly cruise older neighborhoods, those passed over by chains, looking for ethnic finds, mobile or not. Maybe I need to be cruising the Galleria area, Memorial Villages or First Colony, looking for more upscale offerings like these?
Here's more on Holy Cacao.
Actually I think the concept is not so new nor so uniquely 'Austin.' I remember reading a few years ago (at least 2005 or before) about the taco trucks of LA and some of them sounded pretty upscale to me.
Roadfood.com has done a number of features on NYC taco trucks and food carts; here's one on the Treats Truck.