Via somebody on a mailing list, Eric Whiteacre’s virtual choir:
The post I got this from doesn’t contain any details, nor does it contain useful links to the making of this particular video, but looking around the top level of the blog it’s fairly clear that this was put together from a large number of individual videos of people singing just one part of the song. He’s got another piece underway, and you can see some of the individual parts.
This is one of those really cool and impossible-to-predict things you get with the modern Internet. And I think this stuff is ultimately a lot cooler than anything coming out of the blogging-as-journalism model.