UBISOFT announced Assassin's Creed III PC delay quite a few months ago. Recently on their Twitter status, Assassin's Creed dev said the delay is because of delivering better textures, shaders and DX11 tessellation with Assassin's Creed III.
Assassin's Creed III has the following two tweets regarding this.
#1 Not delayed... we just need more time to deliver all those higher-rez textures, DX11 shaders and tessellation!
#2 you get better textures, shaders and DX11 tessellation. Perfection takes time.
Assassin's Creed III PC version scheduled for a November 20, 2012 release.