EDIT: July 31st 2011
Warcraft vs Starcraft's Beta is available now! In the Beta you can play as the Zerg against a random Warcraft race AI!
1. Place "WarcraftVsStarcraft.w3n" in your Warcraft/Campaigns folder.
2. Run Frozen Throne
3. Go to Singleplayer > Custom Campaigns > Warcraft vs Starcraft Beta