Unfortunally, a quite annoying default has been introduced in Debian/squeeze that breaks the Sun Plugin - this can easily be fixed with:
sudo sed -i 's/net.ipv6.bindv6only\ =\ 1/net.ipv6.bindv6only\ =\ 0/' \ > /etc/sysctl.d/bindv6only.conf && sudo invoke-rc.d procps restart
Documented in Debian Bug report logs - #560044
net.ipv6.bindv6only=1 breaks java networking
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=560044