Presence or Absence?

I seem to be bombarded with companies wanting to talk about "federated presence" at the moment. There seem to be 100s of variations on a central theme:

"People have too many phone numbers / devices / services / IM accounts / all of these. Nobody can ever reach them without leaving messages like "I'll call you on your mobile instead" or "u there?" How can we simplify this & make some money as well?"

There are operator-based solutions, web-based ones, PC-based ones, handset-based ones, enterprise collaboration-based ones and IP-PBX based ones. Some of them rely on the user to update their presence information, others infer it from factors such as whether your mobile phone is switched on, your Skype client online and so on.

The idea is that you (or your service provider) should be able to set rules.... "don't phone after 8pm with work inquiries unless you're my boss"... "don't phone me when I'm travelling in a different timezone & it's 3am"... "I'm on a phone call, but email me if it's urgent"... "send my email headers to me via SMS except when I'm logged into MSN in which case IM me instead", or whatever.

All this is great in principle. But I have my doubts about just how viable this wonderful "unified presence" concept really is. Can people really be bothered to update their presence religiously? It doesn't "degrade gracefully" with non-compliance.

I reckon that if 90% of the users use the system 90% of the time, it'll be great.
If it's 80% of people & 80% of time, it'll be marginally useful.
But if it's 70% of people complying 70% of the time, it'll be an active pain in the backside & create more problems than it solves.

And if you're really important, do you actually care if other people have to jump through hoops to find you? Surely that's their problem, not yours?

Maybe "absence" will be simpler and more important then "presence". Rather than try & tell people the "best" way to reach you, why not just tell them which is worst? "Don't both calling my mobile, I'm on a plane". "I've got 1000 emails to reply to. Try something else."

Overall - it all sounds utopian. But I have a sneaking feeling that practicalities may get in the way rather a lot. The user interface is going to a major stumbling block, even if the back-end technology works.