This is going to be markup language, Schema / MicroData perhaps, but I haven't come across this myself yet.
I think perhaps it is time to do some digging
-Andy
Edit
Just read this:
"Update: A Google spokesperson let us know that restaurant menu data is coming from an unnamed data partner. It's similar to how Google uses data from The Weather Channel and other partners to pull in data for searches about the weather."
So there we go - I am sure we will all find out soon.
-Andy