Pronunciation of "因为" 因为 is usually pronounced as yīn wèi, but sometimes I hear it pronounced as yīn wéi which really brings out the meaning as reason (yīn) being (wéi).

Which pronunciation came about first? Do regional differences account for this phenomenon? Are the two completely interchangeable in meaning?
I wouldn't have believed you unless I checked it myself, but it appears these are both interchangeable with one source claiming:

http://www.askbennychinese.com/questions/detail/3713


  In fact, in the south of China, people says “yīn wèi”, while in the
  north people say “yīn wéi”. The only difference lies in the
  pronunciation. Basically, they are exactly the same.


My wife and most of my friends are from the South which is maybe why I have always heard it as yīn wèi.

http://en.wiktionary.org/wiki/%E5%9B%A0%E4%B8%BA says:


  因为 (simplified, Pinyin yīnwèi or yīnwéi, traditional 因為)