Spaceflightnow is carrying notice that NASA is finally retiring its venerable B-52B research aircraft. Used exclusively for launching test aircraft for the past 45 years, it is the oldest flyable B-52 left and (ironically) the one with the lowest flight-hours.
Air & Space magazine detailed its planned retirement at least two years ago (Wikipedia reports they took delivery of its replacement in 2001). The biggest problem is parts... while it is still a B-52, it's so old it has very little in common with modern* examples. As I recall, they had a hatch failure at one point and had to go to a museum to get a replacement from a display aircraft.
In spite of that, NASA was still having a hard time justifying replacing the thing because converting a newer B-52H was going to be very expensive. Plus, as noted above, 008 (the B-52B's tail number) was by far the lowest-time B-52 on anyone's inventory.
But it does seem that they finally got the money/time/incentive to make the replacement, and last month's hypersonic missions will be 008's last. Fortunately, it seems 008 will be put on permanent display at Edwards AFB. Designed for holocaust, used for peace, it will spend the rest of its days where most people would want to be... basking quietly in the California sun.
* A relative term, as the last "modern" example rolled off the line in 1962. They're already being flown by people half their age, and the Air Force has plans to use them operationally until at least 2050. If it chooses to extend this service life further, there's a very real chance the B-52 will be one of the first (only?) modern weapons systems in use operationally for a century.