As far as I am concerned, these things are all up for discussion. As this project is in a very early state and I want to get as many people on board as possible, I feel like I should definitely not rule out options. It is not in the nature of open source and I don't think I know it all. The Arduino is the choice for the first version because it is widely available and supported up to the point that any manufacturer wants to make it's boards 'arduino compatible'.
I have the tools to do small production runs, so making a dedicated board which is as versatile as possible is something I can get behind. The reason I am starting with an Arduino is compatibility with existing hardware. I think a shield with connectors for DS18B20's, JTAG, LCD, relays, SSR's, PT-100 sensors (optional amplifiers on board), etc. can really make BrewPi plug and play. I do want to design this board with the community, so I'll open the forums after the first release.
I'll release the first version for Arduino, with a project for Atmel Studio 6. So if you have your own AVR dragon or similar, you can program bare chips as well.
I don't really know about the DigiSpark. It is very limited in the number of inputs/outputs, program memory and data memory. I'd rather have a full size AVR with JTAG debug possibilities for just a couple of bucks more.
My mailman is slow with my Arduino Leonardo, so I haven't been able to test the new version yet. Should have arrived yesterday
But the Pi is up and running on WiFi already and I feel the two will get along just fine.