When developing rich clients with Java (maybe with JDNC) for the Internet, you need to worry about deploying your application (using Java Web Start, Applets, or some home-made deployment) and making sure an appropriate JVM is ready to execute your app.
Given the number of non-standard Microsoft VMs still around and the fact that versions of Microsoft Windows do not ship a JRE (unless you bought a computer from one of the OEMs provinding it), a few things can go wrong when a user tries to start your application.
Sun has put a lot of effort into making sure Java 5.0 is as compatible as possible with every Java code that’s been written in the past. There’s also a description of the issues you may face while moving off of MSVM, a tool to asses a deployed applet and a forum.
In addition, Sun is providing consumer support for users of the Sun JVM. So your customers can ask questions about the issues they face right here (et ici en français) so that you only get the application-specific questions. As Norbert explains it, this is effective and L10N does matter.