This is very much in early alpha stage today - tech savvy, barbeque, ice-cream & crème brûlée cart-wielding chef-geeks send off tweets in a fairly curt shorthand to their followers, who then turn up salivating, with wallets open. 

This turns the hoary advertising adage of "I'm wasting half my advertising money, but I don't know which half" on its head. These guys aren't spending any money on advertisting, and they know exactly what is being wasted - none of it. It all goes to existing customers. 

I think that next steps in this evolution will be particularly interesting in three areas: First,  semantic data, such as machine readable location info for the trucks, or - and probably not done today - a markup to describe food that's on offer. Second, integrating to mobile, so you can more easily publish (e.g. upload content) and subscribe (integration of your favourite twitterers into your core mobile apps). And third, perhaps the toughest - a business model that extracts some of the value accruing from the vendors and amplifies it, to make a professional service, rather than the tasty, but hacked and ugly mashup it is becoming today.