Previously, "In Rainbows" was available as a free or "pay what you want" download directly from the band. While the success of this promotion has been debated, it was always clearly devised as a promotion nonetheless. Shortly before launching the "In Rainbows" download promotion/experiment, Radiohead announced that the record, at that time, would not be released via iTunes. So what has changed? Well, the biggest change is that Radiohead is no longer with EMI. Digital sales have long been a point of contention between the band and their former label (and it is a primary reason the back catalog is not on iTunes and will probably not be on iTunes in the foreseeable future), now that the band has control over its own music and licensing terms (and is releasing the album via independent labels online and in retail stores), iTunes has become a viable distribution method.
To make it even more clear: Radiohead decided to split from their large record label, in order to build-up hype for the album's official charting release, they did the free/pay-what-you-want online promotion. Now that the album has been officially released (so that it can be tracked by SoundScan and other technologies), it is being made available through both online and retail outlets. Edit: Clearly this is conjecture on my part, based on information released by the band and the music press. For instance, the album is also available at Amazon.com's DRM-free MP3 store for $7.99 US. Amazon also carries the majority of the EMI back-catalog (excluding "Kid A," for reasons unclear to me), which I assume is the result of differing contracts between the two digital services.
Thanks to everyone who sent this in.