There's been a lot of general misinformation on this (and other) supersonic threads. It is interesting to look at current research into eliminating sonic booms, with the intent of over land supersonic travel.
A sonic boom is a pressure wave, much like sound, and it is measured in the same way, as a change in pressure. Like sound, it is attenuated by distance, which in short hand means the further away you are, the quieter it is. No surprises so far.
There are two ways to increase the distance between the aircraft and the observer. The obvious way is to fly higher; the less obvious way is to fly faster. The shock wave has an angle which depends upon the speed; at higher speeds the shock wave angles further back, it extends much further before it hits the ground, hence a quieter shock wave.
Another way is to reduce the intensity of the source shock wave. Clever computers, aircraft profile and even microwaves to heat the air (hotter air = higher local speed of sound = less energy in the shock) have all been looked at.
Technology today probably allows a small aircraft to fly supersonically over land without anyone noticing. The SR-71 replacement (Aurora) almost certainly is a flying embodiment of this technology. Whether it is applicable to Concorde Mark II is harder to say. The speed required to angle the shock wave enough may be excessive - anything greater than M2.2 needs higher temperature metals than aluminium (this is the reason for this design point in Concorde and many military aircraft), which means further expense.
Maybe we have to wait until materials technology allows speeds of M3 to M5 at altiutudes of ~100k. This profile, even today, means no shock wave will reach the ground.
Engine technology - another major hurdle. I'll leave this for someone else.
[ 10 December 2001: Message edited by: Chris ]