We don't have any plans of open sourcing it at the moment. We want it to be a service and it is free. Yup we are starting with slack and we have plans to integrate Chat apps which has the API option open :)
Have we not learned our lessons from how Twitter treated developers?
Enjoy getting shut off when Slack decide they don't want customers to be adding functionality without paying their per integration rate, or want to stop getting the support tickets from people using your service and confusing it with Slack itself.
If Slack pulls the plug on your integration and enough of your income is from users who are integrating with Slack, your business could indeed be impacted.