mike wrote:What is official Muslim teaching concerning Muslims who convert to Christianity?
This is like asking what was the official Catholic teaching concerning Catholics who converted to Anabaptism around 1600. The Catholic position was in clear violation of the New Testament, as much of the Muslim position today is in violation of the Quran. There is no basis in the Quran for punishment for apostasy.
You have been taken for a bit of a ride here by whoever is telling you this stuff.
While it may be true that there is no basis for such punishments in the Quran, there certainly is in Islam. The Quran represents only one in three parts of Islamic teaching.
The most commonly missed part of Islamic teaching for westerners is the Hadith. These are the collected sayings of Muhammed, and are regarded as fully authoritative in sunni islam, which represents the vast majority. They will typically include a "chain of transmission" which is a list of those it was passed through, it is essential that they had to represent an unbroken chain of transmission for a Hadith to be regarded as authoritative, than the body of the Hadith (or saying) itself. There are a number of there collections, the largest is AlBukari, which is about the size of a small set of encyclopedias.
The third source, is the "Sunnah" or example of Muhammed's life, a compendium of his purported deeds.
Most of the penalties for apostasy are in the Hadith. Some examples follow:
Some Zanadiqa (atheists) were brought to `Ali and he burnt them. The news of this event, reached Ibn `Abbas who said, "If I had been in his place, I would not have burnt them, as Allah's Messenger forbade it, saying, 'Do not punish anybody with Allah's punishment (fire).' I would have killed them according to the statement of Allah's Messenger, 'Whoever changed his Islamic religion, then kill him.'"
Dar-us-Salam reference Sahih al-Bukhari Hadith 6922
In-book reference Book 88, Hadith 5
It was narrated from Ibn`Abbas that the Messenger of Allah said:
“Whoever changes his religion, execute him.” (Sahih)
English reference : Vol. 3, Book 20, Hadith 2535
Arabic reference : Book 20, Hadith 2632
Narrated Abu Musa:
A man embraced Islam and then reverted back to Judaism. Mu`adh bin Jabal came and saw the man with Abu Musa. Mu`adh asked, "What is wrong with this (man)?" Abu Musa replied, "He embraced Islam and then reverted back to Judaism." Mu`adh said, "I will not sit down unless you kill him (as it is) the verdict of Allah and His Apostle.
Dar-us-Salam reference Sahih al-Bukhari Hadith 7157
In-book reference Book 93, Hadith 21
Behold: There was a fettered man beside Abu Muisa. Mu`adh asked, "Who is this (man)?" Abu Muisa said, "He was a Jew and became a Muslim and then reverted back to Judaism." Then Abu Muisa requested Mu`adh to sit down but Mu`adh said, "I will not sit down till he has been killed. This is the judgment of Allah and His Apostle (for such cases) and repeated it thrice. Then Abu Musa ordered that the man be killed, and he was killed. Abu Musa added, "Then we discussed the night prayers and one of us said, 'I pray and sleep, and I hope that Allah will reward me for my sleep as well as for my prayers.'"
Dar-us-Salam reference Sahih al-Bukhari Hadith 6923
In-book reference Book 88, Hadith 6
So to say there is no punishment for Apostasy in Islam is simply not being truthful with you.