Sorry but I figure you guys have to have solved an issue that I am facing…
My company e-commerce site is finnally to a point that we want to force our IE customers to use a JVM that is newer than 1.1.8.
We want something that will painlessly and unobtruesively download and install a java plugin in to thier web browser.
I have heard some rumors that webstart will enable a customers to update the jvm that thier browser uses.
Thanks to anyone whom can shed light on the subject
Not quite as easily as it looks. Step 1: make a 1.1.8 compatible applet that will check the VM version and if it’s unsuitable, redirect users to a page explaining why not, with a direct link to Sun’s installer.
Step 2: if VM is correct version, launch Webstart by opening a .jnlp document.
Cas 
With an applet, you can specify which version of Java it requres in the applet tag. You can replace the applet tag with a tag that calls the java-plugin if you prefer, this gives you a bit more power and flexability.
Check out these links on the Sun site for migrating from MSJVM to SunJVM and how to use the plugin to load a newer version of Java onto a clients PC.
http://java.sun.com/j2se/1.4.2/docs/guide/deployment/deployment-guide/upgrade-guide/index.html
http://java.sun.com/products/jfc/tsc/articles/converter/converter.html
http://java.sun.com/products/plugin/versions.html
Hope this helps,
Andy.