Opt in, I reckon. However, every effort should be made to make opting in really really easy. I think I mentioned it in a similar thread previously, but Australia (or at least NSW) has an opt in system, where you simply tick a form on a box when applying for a drivers licence (although PieceOfMind, IIRC, says that that doesn't act as an absolute opt in, or something, but that is at least the general idea). Simple and easy, without the risk of infringing on the traditions/beliefs/desires of those who accidentally fail to opt out under such a system.