Descending to FL600

I recently read a funny (and possibly apocryphal) story about the Lockheed SR-71 Blackbird, and found one of my pictures of the Steven F. Udvar-Hazy Center to accompany it.  An air-traffic controller (American, British, Japanese, etc.) is monitoring traffic and gets a call from an aircraft.  The conversation goes something like this:

Aircraft: Control*, Aspen 21** requesting FL600.*
Control: Roger, if you can climb that high, you can have FL600.
Aircraft: Roger, descending to FL600.
Control: !?

Since the Blackbird could exceed 85,000 feet, descending to FL600 could have required as much of a descent as what many commercial aircraft perform to land.  As with many other aviation anecdotes such as the maintenance-request lists, it may be a combination of several pilots' and controllers' experiences.  If anyone in the aviation community knows any variants on this classic, feel free to post them here.

* Insert London, Shannon, Okinawa, Tokyo, Los Angeles, or other ATC center.
** The Blackbird always uses a generic-sounding call sign in this story.
***FL600 = 60,000 feet, as flight levels are announced as multiples of 100.