Here
Bill Sweetman has written extinsively about the stealth program. He also writes articles for Popular Science. He doesn't say he knows for sure, but presents evidence to prove his point of it's existence. i.e. Radar captures from LAX, Pentagon funding items named aurora
I can't say for sure but I believe it exists because of this reason;
Our spy sats have great coverege, but not total. If a military hot spot or dispute arises, the NRO may have to wait to shift a sat's orbit or for a pass over the area. With something like the SR-71 or Aurora, it can pinpoint a particuler area that may not have good sat coverage. I don't think America would have given up that ability so easily with the retirement of the Blackbird.
Just my opinion.
The website above is a kook's alien site, but it's the only site I could find with pictures from Sweetman's book.
Space/Science » in reply to re: About that X-43 being the 'first' air-breather to reach Mach-10.
Bill Sweetman's Aurora Diagram