Pragmatism and enlightened self-interest should be the guiding principles here. We currently run an opt-in system, have done for many years. Despite this, we still find ourselves with a waiting list of approximately 8-9,000 cases a year, and an ability to only perform transplants on around 3,500 or so a year. Given that for some on the list to need a transplant at all,they are likely to be seriously ill, with a degraded quality of life and reduced life expectancy, a substantial proportion of those awaiting a transplant will die befoer they ever get one.
As I think it was evianbaby pointed out, most people, when asked, say that they support the concept of a donor programme, and find it worthy that people would carry a card - many more might elect to become donors, if they were chased - its like with many other things, the everyday things of life get in the way, and so the donor population remains largely static, despite pretty much everyone being aware of it.
On balance, a well advertised change to an opt out scheme, with plenty of notice given, seems a good idea to me.
There is a lot of work going into synthetic or artificial organs which look very promising in the medium term so such requirements as a donor registration scheme might soon be a thing of the past anyway.