I bought my MBP specifically so it would do everything I need. It's big and powerful enough to act as a desktop replacement. But it's thin and light enough -- compared to my 2+" thick, 11# Dell XPS "notebook" -- to travel easily.
I can run necessary Windows work applications in Parallels. I can play games with BootCamp. I can watch movies or listen to music with FrontRow. I can do photo editing and work (Word, Excel, Powerpoint, etc.) in Mac OS X. I can browse the web and read email in OS X with greater safety and security without the need for a dozen anti-spam, anti-virus, anti-spyware, anti-malware, anti-whatever programs running in the background.
When I'm at home, I can plug in a keyboard, mouse, my 24" widescreen monitor, my big FireWire external HDD, and work with a system that is bette than the "average" desktop being sold today.
All on one computer.
Now, having said that, I see a Mac Pro in my future.