Will Tablets and Mobile Stuff replace them? In many respects certainly, but until a Mobile Platform can build and compile their own applications. Frankly who the hell would want to write software on a mobile platform and watch the battery scream as the app is compiled? I do love my dual screens for development (true laptops can do this, but it's much roomier on a desktop).
Laptops still have a few disadvantages over desktops:
1) You just can't cram as much hardware in a laptop. I want 2 high power video cards , 192GB RAM, 2 Network Cards, 8 drives and 4 physical CPU's, sure send the bill to my company :-p no laptop can fit this and still be called a laptop.
2) Even if you could cram some of it, Heat would play hell....there's only so much you can do to cool laptop casing compared to desktop casing, again laptops will never hold as much hardware
Anyways I've made my point.