From the comments at chopper2004's link
The SR-71 was used over contested airspaces, with an estimated 3000 missiles fired at it. This ought to suggest that early Russian radar systems could see the plane, but the missiles could not catch it. Russian systems now in place would not have any such problems, which is the ultimate reason for the retirement
DARPA signs a contract with Reaction Engines to evaluate the precooler technology of the SABRE engine, including building 'a high-temperature airflow evaluation facility', and Skunk Works hints at Mach 5+ SR-72.
Add in that AFRL validated the concept two years ago and what comes to mind?
A lot of high tech jobs.