When trying to start JBoss from a path that contains Unicode characters the server fails to boot at all.
JBoss 4.2.3 throws an Exception with "ERROR: it turns out the root cause is not always a runtime exception!" from org.jboss.mx.server.MBeanServerImpl.
JBoss 5.1.0 is a bit better:

20:31:25,313 ERROR [AbstractKernelController] Error installing to Instantiated: name=TransactionManager state=Described
java.lang.ExceptionInInitializerError: java.net.MalformedURLException: no protocol: /home/tobi/Vorträge/jboss-5.1.0.GA/server/default/conf/jbossts-properties.xml
at com.arjuna.ats.arjuna.common.arjPropertyManager.<clinit>(arjPropertyManager.java:70)
at com.arjuna.ats.jbossatx.jta.TransactionManagerService.<clinit>(TransactionManagerService.java:110)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:532)
at org.jboss.reflect.plugins.introspection.ReflectionUtils.newInstance(ReflectionUtils.java:149)
...