Blizzard added all WarCraft III models to StarCraft II. They are available on the Public Test Realm (PTR). In addition to the WC3 models, Blizzard added a couple of improved models with higher quality. Some of them could have already been seen on BlizzCon 2013.

The update of ~600mb contains nearly all models and sounds from WarCraft III and its Expansion The Frozen Throne. This huge asset pack now makes it easier for modders to create fantasy maps or better remakes of WC3 classics. The benefit of this coming from Blizzard is that it comes directly from Blizzard.

The Chinese modder Renee aleready updated and published her own WC3 remake with these new models. It already contains nearly all models. You can test it in the PTR's Arcade on Lost Temple. There are a few bugs and missing features, but updates will surely follow in near future.

Unfortunately, this mod also shows that the engines of SC2 and WC3 are very different. Right now, it is impossible to create a map where you play as Zerg against the Undead race from WarCraft as both use the same creep implementation and there is only one creep type possible. This means that a full mix of these games is not possible. In addition to that, SC2 has no naval pathing support, so there is no easy way to add ships pathing through water to your maps.
If you play Renee's map, you might notice that the FPS of the game aren't great. The reason for that is most likely the amount of trees on a map. SC2 has no resource-efficient possibility to add a tree resulting in low FPS.
Furthermore, battle.net has an old problem of not supporting multiple races in the lobbies making it difficult to add a proper selection for a fourth or more races. But Blizzard agreed to look into this problem. This will definetly be a big benefit for extension mods which could then properly add new races with their own alert sounds and UI without workarounds that alters the UI of the other races.

If you want to check out all models yourself, here is a guide:

• Install/Update the PTR using your battle.net App
  
• Start the PTR editor, open a file on the Americas server and search for "Warcraft"
  
• Find and open the "Warcraft III (Data Mod)"
  
• To look at all models, go to Window -> Console, then enter "browse" into it
  
• Use the list view in the new dialog window and search for ".m3"
  
• Scroll down until you see the units whose names start with "War3_"
  
• Click on the green arrow at the top of the dialog to open the preview panel
  
• Use the arrow keys on your keyboard to browse through the list
  

Alternatively, you can add the mod to your map. You can even add Renee's mod to your map as it is contains much more complete unit and spell implementations.

Blizzard stated that they want to keep the PTR up for two months before releasing the mod to the normal servers. This gives them a little bit of time to fix the biggest issues.

Links:
Blizzard's news post.
Blue Post about the PTR duration.