The upcoming launch of Apple's full-on cloud service,, is both a validation that "the cloud" is where consumer media and apps are moving, and a shot across the bow of cloud service companies. Here's how existing cloud-enabled apps are likely to fare.
Amazon's Cloud Player and Google Music both let you store your music in the cloud, but only Apple bypasses the lengthy upload process for tracks you already have by allowing you to send just fingerprints of prepurchased music. Also, Apple's iTunes neatly synchronizes your music with all your iTunes-equipped devices.
Google Music has a slight edge over Amazon owing to strong integration on Android-equipped devices. But from what we know so far, it looks like iTunes' device and cloud integration has raised the bar again for competing music services. The one thing Apple doesn't yet offer, though, is a way to play your music through the browser. As, Apple is using the cloud as a conduit for your data, but end-user access to that data from the Web is not a part of the strategy.
Apple is adding a level of intelligence to online photo storage with iCloud. All the photos you take on your iPhone will automatically appear on your iCloud online Photo Stream for a period of time, and get copied over to your laptop and desktop computers for perpetual storage.
For sharing snapshots among your own devices, it appears that iCloud will be easier to manage than other cloud-based photo apps, like Google's Picasa Web or Yahoo's Flickr. However, there's still no word of what Apple is doing to replace MobileMe, which, among other things, could serve as a stable image host for photos you wanted to park on the Web forever to share with people.
Apple's addition of Twitter integration may also cut into the growth of popular click-and-share services for the iPhone, like Instagram, and possibly Twitter's own new photo-sharing service (though Twitter overall does win either way here).
iCloud backs up all data on an iOS device, but it's not a full-on desktop backup app (like Mozy) or a file synchronization service (like DropBox). Those apps are safe for the time being, but Apple is providing 5GB of free storage to users, which is more than other cloud storage apps (2GB for most services). It may not be long until Apple relaunches an online disk drive for general-purpose cloud storage. (However, the iDisk feature in MobileMe appears to be now offline.)
iCloud keeps e-mail, schedules, and contacts synchronized across Apple devices, and will eventually likely integrate with an online service for Web-based access, though when that may happen is unclear, and Apple has announced that it's killing off MobileMe. Google service users get the same device-to-device sync features, in addition to very strong Web front-ends to that data. Android users are still ahead in this race.
Many modern productivity apps, such as Evernote, seamlessly replicate the data they store to the cloud, which gives users access to their data from any device or Web browser. Apple is adding this feature to its iOS and OS X productivity apps like Pages, and is encouraging software vendors to use its cloud storage APIs in their own apps. So for productivity software vendors, rather than casting a pall over existing apps, in this case Apple is encouraging developers to update their apps to use its own cloud storage and ride along on Apple's cloud coattails. Not all productivity services are safe, though. A new Safari "read later" feature will undo the need for InstaPaper on the iPhone.
To wrap up, it appears that Apple's iCloud will have the biggest effect on the music ecosystem, likely keeping iTunes ahead of competing music stores and services. iCloud could also have large impacts on online photo-sharing, especially if Apple adds a Web sharing element to Photostream. In online storage, communication, and productivity, Apple's new cloud services look very good--but not revolutionary.