I have thought about the privating messaging aspect. We're on the fence about that to be honest because it starts to take away from the public forum aspect, however, I think in some instances, it may actually be a good idea.
Also, we have considered an option to allow the poster to set the message so that
A) People's thumbs will show
B) Anonymous thumbs
The choice will be by the author on the message and the message will clearly show that "Your thubs up or down will be displayed"