Very neat and a good idea. It's probably pretty inexpensive or will be at some point and if I didn't have a co2 tank and carbonator cap (you know, like the other millions of people in the us who don't make their own beer or sodas), then I would get one. Before I got into this, I definately would have gotten one, since I drink slowly and would keep it upside down in the fridge like that. (I'd probably guess the price at 10-20 bucks, and with enough of a production run, 9.99 meaning that it would be accessable to everyone who normally drinks soda but doesn't get all the equipment we do.)
I do expect the soda to be somewhat flatter towards the end, but not totally flat. It's a natural progression of equilibrium, and happens when I dispense soda or beer from my kegs and don't have the co2 going, but with 2 liters (or less, since this would work with smaller bottles that have the same thread diameter) I don't think it would be noticiable.
You know what... In the other thread, how we were talking about a seltzer recarb and losing co2 (in which I take to mean the constant opening and closing and venting of co2), I'd still probably get one, but I'd probably spray paint most of it and make a cover for it. Or as alchemedes said, try to make one or two. And use them when going places with precarbed water. That said... I could probably do the same thing with a bottle with a carbonator cap, a ball lock, hose and a picnic tap. Think I'll go sketch some ideas tonight. It's mostly for design, since I'm sure that even if I prop it up on a plain cardboard box as is, it'd cost slightly more to make than buy. With design, I'm probably just coming up with a simple stand (cardboard probably) and a cover of what I might want it to look like. Glue some gears on, that sort of thing.