At first glance, there seems to be little to differentiate regular old video conferencing from what Polycom and others are calling telepresence.
In reality, telepresence is a form of video conferencing that uses higher resolution images, better audio and is usually more flexible. In a telepresence environment, for example, you could view the other end of the conversation on a high-definition screen while you send the other party HD images of your own.
Additionally, the audio quality would be good enough that participants would sound as if they were in the room. The idea is to have the conference with the same realism as if they were there. Think of it as video conferencing on steroids.