Seems this is becoming the new boilerplate pre-launch way to market a major album on iTunes. Off the top of my head I remember that Justin Timberlake and Daft Punk both offered streaming of their albums leading up to the launch day.
I’d be very interested in what sort of metrics the iTunes store is using to show that this is a way boost sales. Perhaps offering the streaming early is iTunes’s way of getting people to the album page, and then while we’re listing they decide to pre-order. Or, maybe it’s not nearly that clever. Maybe since Justin Timberlake did it and then had off-the-charts sales, everyone else is following blindly in his footsteps.