Of course, cloud computing has gotten a bad rap of late, what with the recent Sidekick data loss, which even fostered a lawsuit. One would hope Disney Keychest has a better backup facility than Danger and Microsoft did.
Disney Keychest wouldn't just be limited to digital media your purchased online or via download, either. According to the WSJ report, purchased optical media such as DVDs and Blu-ray could include keys that would unlock your content for Keychest viewing.
Of course, this just outlines another digital divide sort of scenario. This Disney Keychest service, while it sounds great, would be limited to those who had broadband, or at least, a decent 3G connection on their mobile device.
Additionally, let's not forget that as a content provider, Disney might also have a hard time getting its competitors, such as Sony, Warner Bros., Paramount, etc. to agree to sign up with Keychest. After all, DECE (or Digital Entertainment Content Ecosystem) si already working on a similar service, and DECE is headed up by the CTO of Sony Pictures.
One also has to wonder, with devices like the iPhone already straining cellular networks, what would services like Disney Keychest do to the infrastructure?