What about presence?
UC is definitely in the "marketing hype" phase at the moment. There is much confusion as to what it actually means. Much of the technology behind it is not even close to being new. Some of the technology required doesn't really yet exist. An awful lot of what is needed is not even a technology issue at all - ironically the biggest barrier to UC is the fact that the hardware and software vendors and carriers are neither unified nor communicating!
UC won't really be here until Microsoft, IBM, Apple, Nortel, Avaya, Cisco, Mitel, Vodafone, Orange, O2 and dozens of others all have a framework that allows simple and (relatively) cheap interoperability and information sharing. SIP is not the answer, although it is one of the enabling technologies, as is a Services Oriented Architecture. Microsoft and Nortel have started down the road with their "Innovative Communication Alliance" but that's just the first piece of the jigsaw.
In my view, one of the most important aspects of UC hasn't really been discussed in the previous comments. Without it, many of the gripes about the intrusiveness of multiple communications channels are absolutely spot-on. That aspect is the concept of "presence".
With presence and a single controlling entity for all of those communications channels, I as an individual can determine quickly and easily how, whether, when and with whom I communicate.
If I set my presence as "unavailable" then I should *be* unavailable no matter how you might try to communicate with me. I should be able to set up rules that allow *me* to dictate that, for example, between the hours of 1PM and 2PM I am uncontactable by everyone except the boss (and maybe the wife). With UC, that's one setting in one place, not seven, accessible and configurable from whatever device I happen to have with me (my laptop, my PC, my phone, my Blackberry).
I think the idea of UC can actually result in productivity improvements not necessarily by making people "always contactable" but rather by giving the control back to the individual and making that individual's communication preferences available and visible automatically to everyone else. If you know in advance you're not going to be able to get hold of me because my "presence" tells you that then you'll quickly stop wasting your time even trying.
Now whether any of the currently available systems actually give you this level of control is a different matter. The technology is still evolving, but more importantly, the partnerships between the different vendors are still evolving.
If you want the whole vision of UC today I think you'll be out of luck. If you want to dip your toe in the water, have a look at OCS by all means. Will every organisation out there be able to increase productivity through the adoption of UC enough to justify the expense? No, I don't believe so. However some certainly will.
Taking it further, put UC into a contact centre environment. Why force your customers to call you when they may actually want to e-mail or instant message or SMS you yet still get the same level of service? When a contact centre agent picks up the phone, UC makes them unavailable for contact by other means automatically. Some contact centres use IM systems to allow agents to get assistance from colleagues without having to put the customer on hold. Without UC, how do I determine which of my colleagues is available to help me. UC with presence and skills-based routing makes that an automatic process. It's better for me, it's better for my colleague and it's better for my customer.
And for Joe Public? It all comes down to cost and getting something for (nearly) nothing. I think in a few years' time we'll see "UC" service provision for home users as an extension of the social networking phenomenon. Think of it as an automatic social secretary, screening attempts to contact you based on where you are, what you're doing, who they are, what time of day it is, whether you're already "communicating" with someone...that's unified communications for me.
I'm not a UC evangelist, I'm really not. I don't believe UC is ready for prime-time yet. I don't think it's a panacea for all the problems of the modern world. I do however think that when the time is right, used in the right way for the right reasons, used in the right places and available for a price that makes sense, it does have the potential to be a useful tool, both for individuals and for organisations.