Any major work Boeing/NASA were doing in this respect ended in late 1998, well before Sept 11th.
It ended as they were not at all sure if the many environmental and technical hurdles could be beaten and then produce an affordable aircraft.
Worse still for the environmental aspects, they thought that current standards could be met, but what about by the time in would enter service, at that time, around the 2010-15 period, by which time standards would only be tougher.
Personally, I really cannot see that the world is anywhere near ready for a 250-300 seat, 7-8000 mile range, Mach 2.0-2.6 SST.
However, I note the Aerion design for a Mach 1.6 biz-jet, it looks simple, elegant and practical, maybe even a bit stretchable, 18 in a high density biz config could maybe then increase to around 30 pax.
, it would be limited to Mach 0.99 overland, but though encouraging work has been done in sonic boom suppression, legislation actually allowing it would take much longer probably, with lots of legal challenges.
I like the use of a modified but existing powerplant, reminds me of the adoption of the Olympus for Concorde, but the Aerion could do Paris-New York in 4 hours, 15 mins.
Later boom suppressive biz-jets could pave the way for full size SST's, that still leaves emissions at high altitude and making something affordable, that if it worked would still be a niche, a large niche, but one that would drain First/Business pax from subsonics on it's routes, making them less viable.
Which could be a problem.
Concorde was a niche that complemented the majority subsonics on it's routes, a new SST fleet for BA
would not be limited to 7 aircraft.
But we probably won't see a SST before 2020 at the very soonest.