Disclaimer: I am a reddit admin
Here is what I'm hoping to see as the result of the blackout:
* Awareness raised among the users who don't login to the site(a majority of our traffic).
* A day of action which encourages people to contact their representatives.
* Other web properties participating in some form of highly-visible protest. A lot of the big players are considering how far they can go in protest. Hopefully the step we are taking here will give them some encouragement.
Jimmy Wales recently indicated that he is interested in joining us. If Wikipedia joins in a blackout, the message would reach a huge number of people, and will hopefully make a splash in mainstream media and news coverage.