If one enters <hostname>:<port> in the remote connection string in the Connection dialog, the Connect button becomes grayed if port number is >= 32767 or == 0.

Port number 0 is mentioned as a feature in Eamonn McManus's blog:

"JConsole can connect to its own MBeanServer (i.e. the Platform MBeanServer) and see any MBeans there, including MBeans put there by someone else. To do this, use the Remote tab in the connection dialog and specify host localhost and port 0."

(http://weblogs.java.net/blog/emcmanus/archive/2005/09/mustang_jdk_now.html)