We have noticed a couple issues with the Jabber applet under various operating systems:

Mac OS X: The applet works with Firefox 2.0 and up.  Safari works fine. Tested with OS X 10.4.

Red Hat Enterprise Linux 3: The applet will not run under the IBM JRE 1.4.2. Use the Sun JRE 1.4.2 instead.

Red Hat Enterprise Linux 4: The applet should work if you install the IBM JRE which is the package 'java-1.4.2-ibm-plugin' located in the Extras channel on the Red Hat Network. If you continue to have problems, restart your browser after each failed attempt of starting the applet.