I'm giving Apple a year to deliver something and then I'm either getting a Mediasmart or building a NAS based solution.
I've long maintained that the gap in Apple's product line is the home server. It should be an area that they are leading in. Look at all the duplicate hard drives/storage you have to have now - iPod, iPhone, ATV, MacBook, iMac, etc.
Apple's current iTunes strategy is driven by a single library maintained on a (usually) desktop Macintosh in the home. They need to move that library off that one Mac and put it somewhere that multiple devices can share it. Time Capsule is a good start if they were to further expand the feature set. They also need to be able to allow you to backup your centralized library somewhere. I think that is a killer app for Apple.
If they don't have such a product, they can at least integrate server storage into iTunes, ATV, etc. a little better. Then users can add the storage themselves as others have mentioned.