Mudd,
I think you'll find having the 240v relays in close proximity to the arduino troublesome, this issue has been raised with other arduino projects previously, namely Brewtroller and Matho's controller. With my Brauclone I had all HV in one box separate from the arduino itself. I ran power and signals over Cat5 between the boxes. For me I would consider placing relays in closer proximity to the compressor. Also a point to remember is the PI will not power 2 auduinos so your going to need a good quality powered hub, or separate power supplies for each arduino. [ED] I state the power supply issue as the one you have linked is 5v check this
link post 2349
Looks like you got most of the kit to push ahead, keeps us updated.
MB