Are the two pronunciations of 谁 (shéi / shuí) the result of regional differences?
Most of dictionaries include both of them:


  ● 谁
  
  （誰）
  
  shuí ㄕㄨㄟˊ 又 shéi ㄕㄟˊ


According to 说文解字, 


  从言隹聲。示隹切


It seems to be shuí[1], but according to 康熙字典 it has both pronunciations in different books.

(pinyin added by me)


  《五音集韻》是爲切(shéi) 《玉篇》是推切(shuí)，音垂。


In conclusion, both are right. IMO, shéi seems to be used more in spoken language.



[1] 隹 has three pronunciations, zhuī, cuī, wéi, but only zhuī was mentioned in 说文解字. So I believe the pronunciation of 谁 mentioned in 说文解字 should be shuí.