Sorry, I see I didn't reply to the other part of your question. Yes, the plan is currently to put the entire unit inside the wall. It just doesn't produce that much heat to be an issue, I don't think. The most heat would come from the 110->5v transformer.
I'll need both USB ports, since one will be for WiFi and the other will be connected to a USB->serial converter which will connect to the spa controls interface unit. So, I'm thinking maybe the safest way to go would be something like this:
<http://www.amazon.com/2-Port-Travel-Cha ... B002FFG6JC
then use one of the ports to power the Rasp-Pi while the other port helps power the WiFi dongle. I can even wire in a switch on the Rasp-Pi power line in order to easily reset without having to fish it out of the wall. It'll be a web server totally hidden in the wall, controlling the spa and connected to the net via WiFi. Forward port 80 through the house router and you have spa web page control world-wide (with authentication, of course).