About three weeks ago I purchased my first Mac (a MacBook air) and my first Apple product ever (an iPad 2). The week after my MacBook Air arrived the rumours started about a new MacBook Air being released in June. This stung a little but was a suitable introduction to the Apple Reality Distortion Field. The update to Sandybridge would be nice but not essential for my use of the MacBook Air, but the Thunderbolt and the possibility of 1gb ethernet would be very welcome and would address the once short coming of the Air. The limited network IO capacity to get data onto and off of the Air or to use a fileserver as a primary data store.
Since the news that Apple ordered 12 Petabytes of storage there have been rumours of Apple starting a music streaming service. Apple's not known for making just another example of a given product. To think that the Apple would make a simple music streaming service is a disservice to Apple.
The current model of iTunes is one where a desktop is the master of the user's media while ipod/iphone/ipad are all slave devices that are given a subset of the user's media. What if the master media management role were moved to the cloud? Then MacBook Airs with their limited SSD hard drive capacity could have a subset of the media library. Podcast subscriptions would be synced between devices allowing travellers to take only their iPad or low hard drive capacity MacBook Air and be able to download any new podcasts to their portable device instead of the home desktop. Downloading media such as podcasts or movies could be done once to a any of the slave devices and then synced between slave devices. iPads/iPods/iPhones could be used without a PC at all (this is the obvious result that has been discussed by others) allowing the devices to get software updates over the air.
Currently my iTunes library is stored on a file server at home as it's 300GB's is way too much to fit on my MacBook Air's 128GB capacity SSD. This situation utterly prevents me from accessing my library while travelling as the data IO required would make a vpn to access it somewhat painful. Having the iTunes library file and some or all of the media in the cloud would resolve this issue nicely. If, instead of having the media library in the cloud, Castle only have the iTunes metadata library in the cloud this would still allow multiple devices to have and manage a subset of the media files and sync podcast downloading between devices as well as possibly allowing user devices to stream media data between themselves over the network/Internet.
Don't forget that iPhoto and other apps that sync with iOS devices, they could also be part of the Castle cloud service. Will Apple allow iOS/Mac developers to access/store misc app data in this cloud service? Being able to sync between iOS versions of apps and the Mac desktop versions could be very beneficial to some app developers allowing them to provide such synchronization without having to invest in their own server solutions but to use the user's cloud storage instead.
Eagerly looking forward to the announcement of Castle.