Um, you can be detained for up to 48 hours, I believe, for questioning, without being arrested. I believe the law reads that, providing an officer states otherwise, once handcuffs are placed on a person, a "reasonable person" assumes he is under arrest. Now, here is how that would break down:
You are a witness to a crime. You are suspicious towards LE. You also vaguely fit the description of a suspect (be it clothing, race, car, etc). Officers arriving on the scene secure the scene for THEIR safety. They put handcuffs on you, for their protection, and yours, and state as such-making it clear you are NOT under arrest. They ask you for a statement, which you refuse to give. They then have the obligation to take you in for questioning, knowing that they can only hold you for X amount of time. And, knowing you are a witness, refusing to cooperate. Technically, you ARE committing a crime, by harboring information, thus a criminal. So, doing their duty, they will detain & question for as long as the law allows.
what is said during transport, or anywhere else, is all admissable as evidence. Its all voluntary-the Miranda warning isnt in effect, as there isnt an arrest at that point.
I am not a lawyer, by ANY means. But I believe that the court system has already ruled when one is, or is not, under arrest. You do have the right to remain silent-even is you are just being PC'ed. But if it comes out later that you withheld information, then you will be brought up on charges. So, you silence, in the end, could incriminate you.