You could dual-license Arc, like Gambit Scheme does. I believe Gambit uses Apache 2.0 and LGPL 2.1. I hope that the open-sourcing of the language won't cause it to become too stable and end its evolution, like what happened with Java. I think languages should have a breaking-reverse-compatibility release scheduled every five years.

I guess this announcement puts an end to my plan to read everything PG has written about Arc and then write my own implementation.