Presence is the keystone of unified communications. The ability to identify whether or not a given user is available, and what methods of communication might be most effective with that individual at that point in time is the foundation of what enables UC to make users and business processes more efficient and productive. Organizations that deploy Microsoft Unified Communications can federate with partners or customers that also use Microsoft Unified Communications, as well as with public instant messaging providers such as AIM and Yahoo. However, there remains a gap when one company is using Microsoft and a partner or vendor is using Cisco or IBM unified communications. In order to realize the benefits of presence across heterogonous unified communications systems, the UC vendors need to do more to make sure their toys play nicely together. Blair Pleasant talks more about this in this article from UCStrategies.com.