Repro steps:
1) Make a blank GameObject
2) Put a tk2dUIItem script on it
3) Drag that same GameObject into the "Send Message Target" slot on the tk2dUIItem
4) Drag the GameObject into the project pane to make it a prefab
The "Send Message target" slot in the prefab (and in any instances of it) is now null, and can never be changed. I swear it didn't use to do this with previous versions of TK2D...