What NASA’s up to:

“There are three barriers particular to civil supersonic flight; sonic boom, high altitude emissions and airport noise. Of the three, boom is the most significant problem,” said Peter Coen, manager of NASA’s High Speed Project with the agency’s Aeronautics Research Mission Directorate’s Fundamental Aeronautics Program.

There’s a fourth barrier not mentioned: the low L/D, which restricts range and makes for high fuel costs. If that problem doesn’t get solved, it will never become a huge market, and will mostly be restricted to business jets.