Another thing to consider is new power management options. As the world has turned it's focus on cleaner, greener computing, software and hardware companies have been working hard to meet our demands. Microsoft made serious upgrades to power management settings in Windows Vista.
The truth is that laptops tend to use less power than desktops, but that isn't the only consideration when buying green. Check out this general guide to buying a green PC based on power usage. Laptops use batteries. Those batteries are terrible pollutants that often end up in landfills. Also, the lifespan and upgrade-ability of PCs is superior to that of laptops. In theory, that means laptop hardware is disposed of more quickly than desktops.
There are a number of guidelines and certifications now to help you find green laptops or desktops. Energy Star and EPEAT are good places to start. These organizations help make it easier on the consumer to make an informed decision. You can read more about those here.