Collaboration is the killer app on a Tablet PC . A SharePoint Portal configured with Tablet friendly Web-parts and a wireless Wi-fi connection is the alignment that constitutes the sweet-spot for Tablet PC adoption.
Tablet's need to be light weight, easy to carry and easy to hold.
Shortsighted OEM's have created Tablet PC's as beefy laptops and even suggest that they are desktop replacements. My desktop is the center of my home network and a Tablet PC would make a nice second screen when docked next to my large LCD.
The ability to sync and go is the beauty of the Tablet format, you can share, view, talk, listen and collaborate on documents without so much as one keystroke. A pen is more precise then a mouse and a computer is so much more then just a typewriter or adding machine.
With a Wi-Fi connection and a soft-phone who needs an expensive cell phone?
With a 60 gig Hard Drive, six hours of battery life and Windows Media Player 10 who needs an iPod?
Surfing the Web and reading email and blogs doesn't require any keystrokes, but when I want to work on my novel I can easily pullout a tiny keyboard and connect via Bluetooth or USB. The Tablet PC will never replace my desktop PC as a home entertainment center, but when I'm out and about I can be connected and contributing value.