Linuxcnc does have a built in soft PLC based on a fork of Classicladder but I have no idea if it conforms to that standard. It is still called Classicladder in linuxcnc but it is a heavily modified older version of what is available on the Classicladder website. The Classicladder website does mention that it "Try to take into account the IEC61131-3 norm." (But wheather or not that statement would apply to the version used in Linuxcnc is anyone's guess.)
sites.google.com/site/classicladder/home/features
