Update 2: Just brought to my attention that this is a cover of an old Platinum Blonde song from 1983. (Thanks Greg)
Update: It's up on Youtube! Loving it so far. (Thanks Aaron)
From NME: "The Cure's Robert Smith has lent his vocals to the new single by Crystal Castles. The Toronto digi-dance duo are releasing a new version of 'Not In Love' on December 6, with Smith on singing duties. The original version of the song features on the band's self-titled second album, released in May."
Prefix and Gigwise also have articles about this. (Thanks Kate)
Update: You can read the lyrics to the song here and watch a video for the original version here.