I don't know if there's a trend. Desktops still have an advantage as far as ease of upgrading and availability of parts. The vast majority of laptops are limited to RAM and hard drive upgrades. Some can have the CPU and video card upgraded, but that's not the majority.
Laptop motherboards don't conform to one specifc standard in terms of size. With desktops, the vast majority conform to the ATX or mATX standard, so it's inexpensive to pick up a new one. Same for video cards. Sure, yours might come with integrated video, but you can go out and pick one up. Laptops with upgradeable video modules are out there, but there's not one central standard.
PDA's may start to go the way of the dinosaur in lieu of the smartphone, given how many people have cellphones. Why have a PDA and cellphone if you can have both in one? For the people that are swamped with gadgets, this isn't a bad thing.
I don't think we're going to see the desktop go away yet. While, yes, there are some powerful laptops, a comparable desktop is still cheaper not to mention the user-servicable parts are also less. It depends on the needs of the person.
Trend, not really. I mean, if there's some reputable data you've got, fine, but I don't see laptops taking over anytime soon.