For other devices, I fully expect that you'll be able to use your mobileme account name as a key into that database, etc.
But that requires configuration and account set up. The "magical" aspect of Facetime was that it is suppose to "just work". No account set up .... just magically works by pressing the button.
If you have to have a account that means you have to sign up for one.
Conceptually it would more likely be an Apple ID ( iTunes , etc. ) account since they pragmatically make you get one of those when you get the iPod device. However, that suffers from the
same fundamental issue that mobileme also suffers from; device uniqueness. Your Apple ID is associated with multiple devices. Likewise your Mobile me account can also be associated with multiple devices. They need an ID that is associated with one person
and one device. A telephone number has that property. If you have multiple phones, you get multiple phone numbers. That isn't true of mobileme or Apple ID.
They can stick some gyration on top that says "assign this specific device to this Apple ID / Mobileme" account. However, that is configuration and sign up. It is an account set up.
The other issue is adjustments to the contacts/call app. A caller could can have an email addr and phone number. Which one is the facetime suppose to use for lookup? If it is only phone numbers then it knows what entry to pull out of the contact info to invoke a lookup. The other problem is how is this an industry standard. Does everyone have to lookup ID/ip numbers on Apple's directory servers ? That doesn't sound very standard. Phone numbers have routing info built into them. You hand the phone number to the service provider it belongs to and
they find the phone. If have a system where everyone has to ask one entity where the devices are you have a huge security and scaling problem.
One of the pragmatic problems with the
existing video call standards (they already exist but hooked to telephony delivery) is getting calls to connect across providers. If this only works for a single provider it is a pretty dubious internet standard. The purpose of communication standards should be that they are device and provider independent. If can only make calls to devices of the same vendor that's going back to the 1950's of telephone standards.
You can make it all work by just imitating what IM/Chat services do now. Facetime is just yet another balkanizing IM implementation that loops folks in certain cliques into talking with each other. It is not a "talk to anybody" standard.