Its not that we tablet users don't want a slate - its that most people are not manufacturing anything to the specs we want. They think all we want to do is maybe annotate a types word doc like an editor and then send it back, or sign email.
I take notes in class on my tablet PC, I use digital art programs (Corel Painter, Illustrator, etc), browse the web, read books, etc - often in tablet mode. So why do I use a convertible PC? Better specs... oh, and screen protection when in my bag.
So how do you make a good slate? Better specs, especially in the graphics and CPU size. Better screen size/resolution for digital artists and web designers would be a plus. As for a physical keyboard, how about a hard-shell screen cover with a keyboard inside that connects via USB or bluetooth? How hard would that be? Molded plastic, some means of sliding it on/off, and a low-cost keyboard...
Oh, and when in tablet mode is there anything I can't do that a traditional laptop can? I have a virtual keyboard, though I can just write what I want instead. I can more than "half-surf" on the internet or slideshow pics... believe it or not there are several input methods with a tablet. Okay, I can't touch-type... *sniff* Who cares. IF you use the inking capabilities of a laptop, you have one of the most natural ways of entering information.