Telepathy and kopete
The development of KDE 4 keeps going these days. One milestone achieved these days was a working telepathy plugin in kopete. Michaël Larouche showed that it can already be used for text conversation.
Sure, its just text conversation, but it shows that the basic idea is working, and that there is this kind of including possible. The next step is to put this all together with Decibel, and make it working with Voice over IP as well - and maybe also working with Video over IP.
The advantage of using Telepathy is that telepathy is supposed to be a common backend for all VoIP/IM solutions which are available on Linux.