Thanks for your views!
In fact, we are developing a device that will be sold like 20 - 30 units per year. I think the sales volume doesn't really justify the expense of designing a custom board, baking and populating it.
But how does the Arduinos fare against humidity, high/low temperatures, vapours, vibration, electromagnetic noise, etc? Has anyone got it working in such environments?
My next project will put an Arduino Duemilanove working near powerful pumps, valves, and corrosive chemical tanks (like acids and lye/caustic soda). Let's see how it performs! 
Thanks again!
Óscar Sarabando
Software and Automation Engineer
http://flink.formula9.net