I'm looking at my desktop system and thinking about how, when it finally dies, I probably won't replace it. Buying a second laptop makes more sense from a developers' point of view.
In terms of cost, by the time you add a UPS, an LCD, etc., a laptop isn't that much more expensive than a desktop, and the advantages (less noise, less wiring, less power consumption, less space, less heat) outweight the slightly higher initial cost. And if you buy desktop hardware that makes less noise and uses less power and space, you lose any price advantage over a laptop anyway.
As more people move to laptops as their main (or only) computer, there will be a few changes. For example, a lot of folks now have a "computer room" or a "computer area". This won't be needed any more - just like yuu won't need "computer furniture" or "office chairs" - the couch and the bed are more comfy anyway. Shove the home server (if any), printer (if you still trade in dead trees) and scanner in a closet or an out-of-the-way corner of the garage, and you're good.
What will you be doing with the extra room?
[_] My laptop is already my main computer
[_] I'll be making the switch soon
[_] A laptop will never be powerful enough for me
[_] I'm too busy with my Crackberry to bother with a laptop.
[_] Yes, but will it run linux?
[_] There will always be a need for at least one desktop in my home/office/whatever
[_] I can't switch to a laptop - I use my desktop as a space heater during the winter months.
[_] I'm too fat to have a lap, you ignorant clod you!
[_] My cell phone is all the computer I need.
[_] At $10 a dance, laptops are too expensive
[_] CowboyNeal sold me this palmtop and a really good magnifying glass.