Well just a couple of comments:
First of all this post is heading back to the good old A versus B....
No, now back to some FACTS:
100 years ago aerodynimic knowledge was near zero, that's why nobody believed in aviation as we know it today.
Todays aviation is limited by the aerodynamic knowledge, which is very far developed, making the former large jumps ahead in technology smaller and smaller.
Just compare the 1903 aircraft with the ~1933 design of the DC-3: a huge improvement in 30 years. Today we are still flying aircraft based on a 30 year old design (A300, B747, etc), with improvements in details and way smaller.
We are already operating with highly optimized designs, where just small efficiency increases come with a huge price tag, due to the huge amount of research required. Just look at what CFM is investing in the CFM56 series engine just to reduce costs (operating, fuel consumption, etc) by ONE to TWO percent!
So to get back to the A versus B bashing (at least for some of you out there):
The A380 is a conventional airplane, limited AND designed within known aerodynamic laws, just at slightly larger dimensions. On the paper, already in design freeze.
The Sonic Cruiser is a new airplane, limited by known aerodynamic laws, with still UNKNOWN ways of resolving known aerodynamic limitations.
I am always fascinated by new designs, and a lot of airlines are also fascinated by the fact of faster travel, but at the current stage, Boeings project is a mere idea, and aerodynamics won't make an exeption for Boeing.
Boeing might quite well solve some of the problems, but that will take a long time, so the time frame sounds a little to optimistic.
A similar example was the Concorde. Quite many airlines signed for options, but how many really operated them? Just a little increase in operating costs doomed the entire program. And energy costs definately won't be as low (on average) anymore as they were over the last two decades....
Just my 0.02 (whatever currency) worth....