Recently I've made initial releases of the JNLP Loader (WebStart) that I wrote as part of the FV3 user interface project.
The jnlp-loader project adds a stage to the tail of JNLP project's build process for another product after the principal application archive and JNLP descriptor. The application loader is an executable jar file containing only the code of the jnlp-loader with the JNLP descriptor for the application.
A typical loader program (jar file) is a 20 or 30 KB download. It unpacks the platform specific resources and runs the application (main) via a deletable and updateable temporary directory. Most interesting for JNLP / WebStart application developers is the simple java main runtime of the insecure jnlp-loader in comparison with the secure javaws.