Actually, according to the supreme court, you have to verbally invoke your right to remain silent. If you dont, and they harass you for 16 hours and you say anything, its fair game. If you do, and they opt to keep harassing you, its not.

I know what you are getting at, but that recent SCOTUS decision didn't implicitly say that. However, it did make it clear that you can't talk with police (thus waiving your right), refuse to answer a question, and then answer other questions. In essence, it's not the police's job to figure out whether or not you're invoking your right.