Unfortunately, many GNU/Linux distributions (include slackware and Fedora), ship with a gij version that matches Java 1.4.2, and although it has some 1.5 extensions it doesn't seem to be able to read the class file version. This is pretty limiting, IMO, but of course, free software nuts who don't use the Sun JRE may not exactly be your target audience
This is the error I get:
Exception in thread "main" java.lang.ClassFormatError: jifi.Jifi (unrecognized class file version)
at java.lang.VMClassLoader.defineClass(libgcj.so.7rh)
at java.lang.ClassLoader.defineClass(libgcj.so.7rh)
at java.security.SecureClassLoader.defineClass(libgcj .so.7rh)
at java.net.URLClassLoader.findClass(libgcj.so.7rh)
at java.lang.ClassLoader.loadClass(libgcj.so.7rh)
at java.lang.ClassLoader.loadClass(libgcj.so.7rh)
at gnu.java.lang.MainThread.run(libgcj.so.7rh)