Is it illegal to slander someone as guilty when the lawful verdict in court was innocent?

This whole mess with the Casey Anthony trial has caused quite an uproar among many people. There has been a lot of slander on her name. I was curious as to the legality of such slander, or how far people can go with the "She is guilty, I know it" act before it's against the law because in COURT she is innocent? I'd appreciate people's opinions kept to themselves, that's not what I'm asking for. Thanks!