[noisebleed]

Trent Reznor / Nine Inch Nails has done it several times: http://www.ninremixes.com/multitracks.php

Plenty of other artists have as well, but this is the most high profile example I can think of. I agree it would be great if it happened more often.

[polshaw]

I'm not a huge NIN fan, but Trent is truly awesome when it comes to digital music. You can add excellent mastering and dedicated surround mixes too..(rec: Social network soundtrack). Also a former oink'er.

The beatles multi-tracks are also available (although they were only recorded 4-track so not every instrument always has it's own track), and there has been a handful of artists who have released their samples of one song for remix competitions (Daft Punk, Royksopp, Booka Shade).