This picture was taken just as the fighter went supersonic. Gil (an ex-fighter pilot) tells me that the cloud behind it is condensation.
Okay, I stand corrected. The condensation cloud has more to do with the moisture content in the air than breaking the sound barrier, according to Bill.
He also was kind enough to send along a link that not only explains it a hell of a better than I can but also shows more cool pictures.
Here's the first couple of paragraphs of that page:
Here are some fascinating (for some people anyway) photos and videos of interesting condensation clouds that form around jets as they fly at or near the speed of sound, (often called "going through the sound barrier" or "accelerating past the speed of sound"). Under the right conditions, and even at lower speeds, they sometimes cause a vapor cone effect.
Understand that these Prandtl-Glauert condensation clouds can also occur at lower speeds, and are not really a visible manifestation of some kind of a sound barrier being broken.
Thanks again, Bill.