Cintiq 21UXI did it.
I said I didn't see the use -- I already feel 100% natural on the regular Wacom tablet, but watching the demo videos and realizing I could pivot the Cintiq around like an animation board was something I hadn't thought about. I've got a new picture book in the works with old NYC in perspective and I don't use straight edges so I've got to be able to tilt the tablet to draw accurate lines.
Here's how I'm set up, L-shaped desks (actually cheap Ikea dining tables) and my chair with arms removed and set at maximum height. I have a stool under my desk for my feet. I had thought to put the Cintiq straddling both desks (diagonally) and keep the keyboard on the left for typing software shortcuts in Painter/Photoshop -- but decided to go cold turkey and get used to using the Cintiq tabs for the shortcuts I use most often.
My other two concerns in investing in the Cintiq were messing up the screen with my palm (I'm the type to yell "DON'T TOUCH THE SCREEN" if anyone so much as points in the direction of one of my monitors) and secondly, it seems to me that perching bent-necked over a table again is a step backwards in terms of ergonomics. Time will tell. Stay tuned.