The new Java Plugin supports using JRE 1.4.2* or later on Windows, or JRE 1.5.0* or later on Windows. Java Web Start supports using JRE 1.3.1* or later on all platforms.
However, if you request an earlier version, such and applet requesting 1.3.1 on windows or 1.4.2 on Unix, or if a webstart app or plugin applet requests using 1.1* or 1.2*, the code will just attempt to download it, and fail saying it is not available. (or the install may succeed - say you are installing 1.3.1 for an applet)
In all cases if the version is not supported, we should just show an error dialog saying the version is not supported.