Blizzard announced that they have just released the Warcraft III assets to the PTR (Public Test Realm) of StarCraft II Arcade, and that should be followed soon by a general release. If you are not familiar with StarCraft's Arcade mode, it allows users to share their own mods and creations with other users.
This means the modding community will be able to work with "over three thousand new materials", including "new hero models for the 4 primary races, all of the original Warcraft III models including neutral creatures and their sounds as well as doodads, structures, spell effects, ambient sounds, music and custom user interfaces for each race".
Blizzard also promised to spotlight the best user-created content in a new feature called "Memories of War". You can get more details, and info on how to access the PTR servers, here.
The question now is: how long till we see Warcraft III and Frozen Throne recreated with the StarCraft II engine?