Gil, Laptops are a different matter, and I would not leave them on longer than absolutely necessary. Even when you do have laptops on, I recommend using a docking station in order to ensure that the laptop is elevated slightly to prevent overheating. Yes, laptops have a tendency to overheat - even the ones using Pentium M chips which draw significantly less power. Windows XP/2000 also require shut down to fix the many errors that creep up during use.

Desktops have the heat sinks and fans necessary for keeping them on; like the others who've posted on this thread, I keep my desktop on all the time. Each time you turn a device that uses solid-state circuitry on and off, you slightly damage it because the act of turning on a device is in itself a mini power-surge.