In game in Fallout 3, they appear to be slightly enlarged versions of the classic Coke bottle.
The "promotional" Fallout 3 memorabilia, both the real bottle of soda (Has anyone drank it? What does it taste like?) and the can opener are very generic glass bottles with no particular shape whatsoever.
The Nuka Cola icons in the older games are hard to make out, but they both appear to have some definite curve, and do NOT appear to be clones of the Coke bottle. In fact, they seem to almost resemble a Bawls bottle, although that might just be the color.
Does anyone have a better picture of the "old" bottles than the odd perspective icon? Or any other editions of Nuka Cola bottles? (I'm thinking of making a 3D model of one for a high res wallpaper)