I measured an Athalon XP2600 (or thereabouts) recently, and it took 120W when fully booted up but doing very little (Word loaded, but nothing going on). This was without a monitor. Let's add 30W for a TFT monitor and that comes to 150W. I think that a laptop would probably take around 40W under these condtions, including the monitor!
So, let's say that 10 million people replace their desktops with laptops, and they usually use them for two hours a day (some much more, some much less, so this is a wild guess at an average). Each saybe 150 - 30 = 120W. That's quite a bit of electricity.
I suspect that more recent desktop PCs use even more electricity, especially those with wizzy graphics cards.
Interesting point: I also measured a much older desktop recently - a Dell PC with a Pentium III 667MHz processor, internal graphics (no graphics card) and it took just 29W without the monitor under the same conditions.
Power was measured at the mains using an AC power meter, which takes into account things like 'power factor'