Starting with Java SE 7 Update 21 in April 2013, all Java Applets and Web Start Applications should be signed with a trusted certificate.
The April 16th, 2013 Critical Patch Update for Java SE (7u21) will change the launch behavior related to running Java Applets and Web Start Applications. Users will be presented with a dialog(s) that will provide additional information to the user who can choose to continue or terminate execution. For the best possible user experience, your Applets and Web Start Applications should be signed.
What does this mean? Java SE 7u21 will introduce changes to Java browser plug-in behavior, encouraging application authors and vendors to sign code with a certificate from a trusted Certificate Authority. You, as a developer, are strongly encouraged to sign code now in preparation for this release and future releases.
For more information, read Java Applet & Web Start - Code Signing on OTN.