Excelsior has commented on a major change coming in v5 of their Java SE 5 implementation, Excelsior JET. To reduce the download size of applications, developers will be able to exclude parts of the JDK from the application.
Excluded components will be packaged as a single downloadable file that will be placed on the server where your application is available. While the application is running, if it attempts to access the excluded components, the JET Runtime will pull the bundle down from the server. Excelsior describes the situation:
The key idea is to select a few components of the entire Java SE API, which the developers could optionally exclude from the installation if the application does not use them.
For example, if your application does not use any of AWT, Swing, CORBA or, say, JNDI API, you will be able to exclude them from the installation created with the JetPackII tool.
No word yet on how big of an impact this can end up being for a typical application nor when we can expect to see a preview of the functionality.