I'm getting one mainly for one app: VNC.
I've got one headless mac running my music studio. I've been running it by logging on to it with my laptop, but a laptop is a pain in the neck when you're rehearsing, recording, etc., especially if you're standing up most of the time. It also means I don't need to print up lyric sheets if I'm learning a new song, since I can just read them off the iPad screen as easily as a book.
The studio computer is rack-mounted in a road case with my PA amp. Using an iPad, I can run sound for a full band from anywhere in the club with no need for a cable snake. Just garageband, WiFi, a shared desktop, and any laptop or smartphone running VNC. I've been doing it this way for a while and it's AWESOME, and will be even better using an iPad for the controller.
I also have a mac driving my media center. I don't care to turn on my projection screen just to launch iTunes so I can listen to music, so I remote to it. Doing so with a laptop sucks.
For the rare times when I do want to accomplish something that I would normally do on a laptop. (Photoshop? Web design? Video editing), I can just plug it into the keyboard dock, and remotely run one of my other computers, where the "heavy" apps will actually live from now on.
So there's no need for a "desktop replacement" laptop for me anymore. Just an iPad as a thin client to my "real" computers. I honestly can hardly wait.