It hasn’t been confirmed yet, so take the following as a grain of salt.
Today on the dashboard, I saw an ad for Halo Wars for GonD. As I scrolled over to it, it gave me the recommendations which were Halo 4, Halo 3: ODST campaign edition.
This means Microsoft has already registered ODST for Games on Demand, but upon further searching in new releases, coming soon, or A-Z word for word search, I get nothing but trailers and ViDocs.
I’m sure ODST will be on Games on Demand and fairly soon.