Interesting subject.
I too have wondered for years whether or not such a plane exists or is under development. Most supporters of the idea point out that there was no real successor to the SR-71. Some folks credit satellite technology as having obviated the need for spy aircraft, but satellites move in and out of phase, have limited times over targets, and can't always provide real time intelligence. Therefore, the logic follows, an SR-71 like plane is needed.
Ben Rich, in his wonderful book Skunk Works , claims that the code name "Aurora" was assigned to the funding for the B-2 project. However, when that information leaked its way out of congressional hearings, someone in the media ran with the story and, according to Rich, fabricated the story about a hypersonic spy plane being built by Lockheed. He flat out denies the claims, saying that such a plane, "...simply does not exist." (pg. 309)
Perhaps Rich was right and no such plane exists. Maybe, but I'm not so quick to dismiss the possibilty. It could be that Lockheed isn't in the loop, but that doesn't mean that Northrop/Grumman, Boeing, or some other company isn't or hasn't been working on a similar project for years.