Retail version (store bought) version of HL2 does NOT include DoD:S
If you bought a copy of HL2 in a store, you do not automatically get DoD:S once it is released. This goes for all versions, including the collectors edition.
DoD:S is only available for free on release to those who bought the Silver or Gold packages via Steam.
What! This is an outrage! I paid $XXX for HL2! I demand DoD:S!
The store versions of HL2 were duplicated, packaged and distributed by Vivendi who had the publication rights for HL2 and CS:S only. These rights did non include DoD:S which is why it is not availble with the retail version.
How do I get DoD:S when its released then with a retail/store version?
Non Steam Silver Package owners will be able to purchase DoD:S via Steam for $19.95 or in stores by purchasing the Counter-Strike: Source retail package for the same price.
But DoD:S is a mod! It should be free!
DoD:S is not a mod, in fact DoD hasn't been a mod since beta 3.1. The mod was bought by Valve and as of 1.0 (when it was available in stores), it became a commerical, stand-alone product. It is now a game in its own right and not an add-on.