MetaMorph is a Unity3d and Blender3d toolkit that allows mesh animation in Unity games using Blender shapekeys. It includes an export script for Blender that converts shapekeys into Diff maps and saves the animation data as text files. The MetaMorph script for Unity can read the Diffmaps and animation data in order to play back the morph animations in real time.
You can find more information on MetaMorph and get the free download here: http://rezzable.com/metamorph
You can see the demo at http://rezzable.com/FoolishDesigns/MetaMorph%20Web%20Demo/WebPlayer.html.