I am sure a gas guzzling SST would be a non starter. However, with technology advancing it does not need to be gas guzzling - Hydrogen was suggested as a fuel. Yes it would be uneconomical making hydrogen with fossil/nuclear power but if you could use windmills to make the hydrogen then it woudn't be environmentally unfriendly in itself.

The key to negating the boom would also be altitude - a high altitude service might be feasible JFK-SYD - with the right technology perhaps it could even be fuel efficient vs a conventional stratospheric flight?

If China becomes the super economic centre it promises to be then JFK-PVG would be a prime route for time poor city types. How much does video conferencing negate the need for this type of travel?

Perhaps our planet is just too small to need that kind of speed - I have to admit that I preferred flying back from JFK in F as an R flight was over too fast and F is special enough for different reasons. I am not a time poor city type. Do those people really exist?