The announcement is posted at LWN.net: "What is it?
The -jp kernels are development kernels and for testing purpose only. They
will appear regularly two or three times a month. Their purpose is to provide
a service for developers who can't keep up to date with the latest kernel and
patch versions, but want to test new features and evaluate enhancements that
are not to be expected for inclusion into the mainstream 2.4 kernel.