It's called Lux Aeterna. It seems to be a stronger/faster version of it. I haven't searched around a lot, but here is the slower version from Requiem For A Dream that you can listen to:
http://odeo.com/audio/4571323/view
If anyone finds the Lost version, I'd like to know more about it as well.
Edit:
Okay, I found some more info-
Here is a longer Lost clip with the song:
http://www.dailymotion.com/tag/dream...et-lux-aeterna
And apparently it was performed/composed by Clint Mansell. Here is his myspace page, which has the slower version of the song available:
http://profile.myspace.com/index.cfm...endid=29604174