Its the start of the relationship between VALVE and Activision. If you own Half-Life (and a legit CD Key) then just download v1.0 from the usual places. If you dont own Half-Life, then purchase it. Or purchase it to show support and gratitude to the devs of Day of Defeat.
As far as I know, there is no real big difference between v1.0 MOD and v1.0 Retail. Unless VALVE were forced to replace the gun names with other names. (Like Counter-Strike retail)
Take Your Pick...