Blogging in support of Justin, here.
Ideology aside, if most people want to have their organs donated after they die, then we should have an opt-out system out of simplicity. Even with some very strong protections (ie. all it takes is a word from next-of-kin to prevent donation) the number of donations would increase substantially.
And if you opt out of allowing your organs to be donated, I think that it's only fair that you opt out of receiving organ donations from others.