Typically, the priest would be defrocked. I've heard that from just about every source I've encountered on the subject. After that, the bishop could possibly prescribe a penance (temporary excommunication).
As far as when a priest can and can't reveal a confession, and under what circumstances...I don't think there's a universal concensus. I've heard from some churches that a priest can and should never reveal anything in any way. I've heard that if a crime is confessed, the priest can inform authorities.
My priest has spoken about confession a few times, both casually with a group (after service at coffee hour, etc.) and formally at catachism classes, and he says the tradition he's received and follows is a middle-of-the-road approach. He cannot reveal a crime already committed, but would prescribe penance until the person turned him/herself into authorities. If one confesses that they are seriously struggling with commiting a criminial act, but has not yet, he could warn the potential victim(s), without revealing details, and attempt to prevent the act from occuring without breaking the seal of confession.