I would have to agree that the police are not required to contact you about a pending arrest. If they were, how many people would actually be around when the warrant was served?
I, like the other posters, would suggest that you get a lawyer. Obviously the police and district attorney has something, or the arrest warrant would not have been signed.
It sounds like you need to immediately contact an attorney to help you deal with this problem. It could be that someone convinced law enforcement officials that their charges were truthful, and then a warrant was sworn out for your arrest. You may be able to have these charges overturned, and a civil suit against your accuser could be a solution.