Hardware wise it should be really easy and inexpensive to produce. All you would need to do is to is order the part and stick it in a plastic box with a RA logo on it. Then it could attach to the head unit with the USB TTL Converter cable.
Software should be just as easy. The arduino api software can be installed on the pi as easy as "sudo apt-get install arduino"
As far as uses, I can see it being very useful:
- Hard Wired Ethernet
- Data Logger
- Web cam
- Full featured LAMP Web Server
- Load custom code onto the Reef Angel without having to run a long wire over to desktop/laptop. ( plug thumb drive with code into the pi (or you could send the code to the pi via Eithernet), and it could automatically load the code onto the Reef Angel )
Yes, this all of this should be easy enough to be a DIY project, but I'm sure that an official Reef Angel PI would end up a lot nicer.