Well as this article sates, it should be arriving on steam this summer, maybe they'll put it on the "coming soon" games list next to Day of Defeat: Source. BTW when is DOD:Source going to be released? anyone hear anything on that?
HL2: Aftermath's tentative release date is September. I believe Doug Lombardi said this. He also said that you will be able to buy it over the counter. Valve is still looking for a publisher because their contract with Vivendi ends August due to winning lawsuit.
DOD:S I believe will come out July or around same time as Lost Coast. They are putting HDR in DOD:S.