Ey alexei, love your new avatar.
http://answering-islam.org.uk/Silas/apostasy.htm
Pay special heed to this part of the website
The Dictionary of Quranic Terms and Concepts - page 16, (written by M. Mir - a Muslim writer), defines the actual Quranic information as:
"APOSTASY
Arabic "irtidad". Traditional Islamic law prescribes the penalty of death for a Muslim who commits apostasy. The punishment is not stated in the Quran, but is said to be based on certain Hadith. The advocates and the opponents of the said penalty have, in their attempt to find Quranic support for their views, appealed to certain Quranic verses, but the fact is that none of the arguments offered do full justice to the Quranic context......"
As you can see, it's based on a hadith, which are known to be "wrong" or "errornous" and not to be Allah's word the Qu'ran.
It's the turn that muhammed gave to it that it became uhm, legit.(easy way to get rid of opposition, just kill em *grin*)
Michael