Kieran Healy over at CrookedTImber presents evidence that, while opt-in vs. opt-out does make a difference to whether individuals agree to donate, this doesn't necessarily translate into differences in actual organ procurement rates, and argues that the real bottlenecks in many countries are organizational/logistical.

The apparent lesson: Don't assume that just by removing the obvious trivial obstacles, the problem will be solved. There may be less trivial obstacles lurking in the background.

P.S. Reading off the graphs, Austria, Belgium, France, Hungary, Italy, Norway, Poland, Portugal, Spain, Sweden, and Switzerland all appear to have presumed consent.