I'd like to see the synergy of five technologies:
1. Wi-Fi
2. Portable computing
3. VoIP
4. Bluetooth
5. Voice recognition
Imagine having a little PDA on a belt clip with as much power as today's computers. Thanks to Wi-Fi, everywhere you went, there'd be an internet signal. And thanks to VoIP, that computer would also be your cell phone. You'd have your bluetooth handsfree earpiece on, and making a phone call would be a simple matter of telling your computer who to dial.
When you get in your car, you'd plug it into the dash, and it'd fire up the GPS software.
When you got home, you'd plug it into a port replicator, and suddenly that PDA is running your LCD, home theater, dishwasher, and toaster.
The only people who'd have tower computers would be the hardcore gamers and graphics/3D/CAD professionals. Everyone else would just need a little $200 PDA, $50 port replicator, and then monitor, speakers, keyboard, mouse, etc.