I used the provided example (X11Forwarding.java) to connect to several remote machines using different linux distros and tried to run a X application, which failed.
Note that I had to change the port number from xport + 6000 to 6010 + xport, because the openssh uses 6010 as the first port for X-Forwarding.
To do the tests I used JSch 0.1.28 and the following opensshd versions:
OpenSSH_4.3p2 Debian-5ubuntu1, OpenSSL 0.9.8b 04 May 2006
OpenSSH_4.3p2, OpenSSL 0.9.8a 11 Oct 2005
I discarded authorization problems using the "xauth +" command.