A headless Mac to me means a monitor-less Mac to fill the void between the Mini and the Pro. Something with a little better performance than the Mini and a little expansion capability, like the ability to plug in a better graphics card and a 2nd hard drive. Basically an iMac chipset packaged in a smallish box.
A PVR (Personal Video Recorder) would be an AppleTV with a digital tuner so that you could record broadcast TV for free instead of having to buy everything through the iTunes store.
Will Apple Ever Give Us What We Know We Want?