Really interested in this post as I do a good deal of work with PLC's and OPC servers.
OPC is a really good option as it can make alot of different makes of equipment "talk" to each other.
Also you can buy PCB devices that you can get any type of data out via OPC http://embeddedlabs.com/servermodule.html
I am interested in the PLC programs that Fedex Guy has as I would love to see how he is doing this.
A good option I reckon for a PLC would be Allen Bradley Micrologix. It has the serial port and an ethernet port and you can expand the IO with various add-on cards.
I reckon if this method was explored it would be a great advantage to the LinuxMCE system.