Presence is the ability to see other people/colleague/customer availability. Learn more at http://www.microsoft.com/downloads/details.aspx?FamilyId=E297AA65-150E-4B9F-93E9-A313B7988A0E&displaylang=en
Which I half agree with, but there is so much more to it. Here is the summary from the above document:
Rather than slowing work processes and decision-making by trying to contact others when they are unavailable or busy, organizations can improve their efficiency and make faster, better decisions by using the power of presence. When presence information appears in the applications most often used by information workers, it reduces the time needed to answer questions and complete business projects. The ability to understand the preferred communication method of one’s colleagues allows people to collaborate regardless of time zone, location or device. Today, real-time knowledge is dramatically increasing productivity and effectiveness for companies around the world.
If you are sitting on the fence and after some information about presence or looking into LCS 2005 (or waiting for Office Communication Server) I would definitely recommend having a quick read of this document.
Finally if you are looking into a new voip phone system then really please do a bit of reasearch into presence, it adds so much and its such a tiny fraction of the cost of replacing a PABX system that it really is a must!
Any questions, just ask!