Dee Ray sent us an email with a bunch of pictures of jets with vapor cones around them. According to Wilk4, this phenomenon can be seen at lower speeds under the right conditions.
This phenomenon, called the Prandtl-Glauert singularity has to do with rapid changes in temperature and pressure as a plane approaches the speed of sound. Condensation forms because of cooling of the air. This phenomenon is more common as planes fly over the ocean, because of the high humidity. Many great photographs have been taken from Navy carriers.
[Image and details from Wilk4]
[Thanks, Dee Ray]