Desktops are perfectly justifiable for software developers and will stay that way. Hard core gamers are (for now) still stuck to powerful desktop machines but that may likely change with more powerful tablets and new command interfaces and sensors.
But developers will always use desktops or high-end notebooks (for those of us that are on the move most of the time).
Unfortunately this means that desktops (may as well come packed in a very small form factor) will likely get increased prices due to less demand while tablets will be dropping it considerably (which is basically happening these days already).