Hi.
congrats - a great idea and great execution. For industrial use I would suggest adopting a modular approach - the variety (and physical size) of some of the I/O that is going to be popular in this application probably won't fit well inside your enclosure and would want to be electrically (and EMF noise) seperated anyway. e..g I'm thinking maybe 8 Mains rated relay's would be a very common application but tricky to fit inside your enclosure safely to a degree that anyone would be comfortable using in a safety regulated industrial setting. One definite on my wish list though has to be the Ethernet interface. With that and probably i2C and RS232 and RS485 interfaces exposed you would cast the Industruino as the intelligent hub of a solution that could displace many PLC and more expensive microcontroller based solutions.
Other posters referrenced bringing the I/O out on a side connector to allow daisychaining to I/O modules on the DIN rail. Thats a common technique in the industry but one I personally find a little inflexible when one has a lot of I/O as it means you need a long single run of DIN rail which is often awkward to accomodate. extending on ribbon cable might be an option but is harder to suppress interference on low voltage I/O lines so personally I would stick with the individual I/O screw terminals you have and allow people to add on your or others external boards.
I assume you have seen this? http://www.ruggedcircuits.com/ruggeduino/
Have you gone for similar levels of electrical protection for the microcontroller in your design? That is pretty much necessary to install in industrial situations unless you want to end up with a lot of broken units by careless installers 
Great job - will be following with interest and as soon as a board with inbuilt ethernet and maybe some of those serial interfaces is available I'll definitely be buying one to evaluate.
good job.
Guy