I think people are neing alittle too specific in discussing the whole "innocent until proven guilty" aspect.
If someone has a warrant issued for them, and despite knowing of the warrant you assist them in evading that warrant being executed, I think you'd be in hot water - even if they were later deemed innocent. I imagine you could be done for some variant of obstruction of justice and/or contempt of court.
Certainly I wouldn't like to rely on the "I knew there was a warrant but he hadn't been found guilty yet" defence....