I agree with Keith Howell, in that there are several
types of existing products.  A new one out is called LOGO by Siemans,
and I believe one model has an interface to some sort of bus.

                Think about it.  Design and develop the software for
this application.  PC board, with PIC and relays. Enclosure.
Documentation.  Can you afford $100 per node.  If yes, buy something off
the shelf.  If you have unlimited time, money and resources, might be
fun to build.  And this doesn't even talk about the PC side of things.
I suggest FieldBus, and the PC software that supports it.

                BTW, who else out here is doing industrial controls (PLC
and the like).  Might be nice to have a list of those so we can compare
notes and problems away from the list.