It's produced by a shock wave from a plane crossing the speed of sound barrier. The change in pressure causes the water in the atmosphere to condense into a cloud around the plane. They have some more about this here, and probably at some other sites if you google shock waves and planes or something of that sort:
http://images.google.com/imgres?imgurl=http://aeweb.tamu.edu/aeroel/Gallery/Interest/shock.jpg&imgrefurl=http://aeweb.tamu.edu/aeroel/gallery2.html&usg=__JU8uAfkixkxeWl-6RC0g3ufe-4E=&h=367&w=512&sz=11&hl=en&start=17&um=1&itbs=1&tbnid=pwvzPA9o4EO6HM:&tbnh=94&tbnw=131&prev=/images%3Fq%3Dshock%2Bwave%2Bimages%26um%3D1%26hl%3Den%26client%3Dfirefox-a%26sa%3DN%26rls%3Dorg.mozilla:en-US:official%26tbs%3Disch:1
http://images.google.com/imgres?imgurl=http://aeweb.tamu.edu/aeroel/Gallery/Interest/shock.jpg&imgrefurl=http://aeweb.tamu.edu/aeroel/gallery2.html&usg=__JU8uAfkixkxeWl-6RC0g3ufe-4E=&h=367&w=512&sz=11&hl=en&start=17&um=1&itbs=1&tbnid=pwvzPA9o4EO6HM:&tbnh=94&tbnw=131&prev=/images%3Fq%3Dshock%2Bwave%2Bimages%26um%3D1%26hl%3Den%26client%3Dfirefox-a%26sa%3DN%26rls%3Dorg.mozilla:en-US:official%26tbs%3Disch:1