Muslim scholars teach that Muslims should generally be truthful to each other, unless the purpose of lying is to “smooth over differences.”

There are two forms of lying to non-believers that are permitted under certain circumstances, taqiyya and kitman. These circumstances are typically those that advance the cause Islam – in some cases by gaining the trust of non-believers in order to draw out their vulnerability and defeat them.”

http://www.thereligionofpeace.com/Quran/011-taqiyya.htm