Greetings. It is important to note that when deriving evidence in Islam, we must look at the Qur'an and Hadeeth together, never exclusively. It is a major failing to solely rely on the Qur'an.
Hugo - yes so one cannot understand what God himself has said unless we have the hadeeth? I know this to be the Muslim position but it is not a very convincing one is it and leave open the door to preferring the hadeeth to what God has said. Surely, God can be clear?
Another important rule in understanding Islamic law is that the ambiguous (if that is an appropriate word) information is understood in light of the clear - not the other way round. Thus, I wonder how Professor Charfi would understand the following explicit Hadeeth (from the link given by sister Skye),
Hugo - Professor Chafi understands this link very well but he points out that it is the only such link, its is not a strong hadeeth and it was made by someone who was a 12 years old boy at the time he heard it. Hardly enough to start killing people for a change of religion is it?
"
Whoever replaces his religion, execute him" (Bukhari, Abu Dawud)
I think there is a misunderstanding here regarding what Shariah is. I recommend the following video which sheds light on this specific issue - it begins from 3:45 onwards:
http://www.youtube.com/watch?v=p1e0P4P3Rm0&feature=related
And I think the whole lecture (to be found on youtube) may very well be relevant to this thread.
Peace.