Now that a pull request to merge RingCT has appeared in GitHub are there plans to create a public testnet? If so how would someone participate and help test the code?
There will be a public testnet once there is some code review. I'm not sure whether the code will be merged prior to the public testnet or not. If it is, then you will be able to pull from master and run in testnet mode. If not, you will have to build from moneromooo's branch. From today's dev meeting, the testnet should happen a few days after the PR receives basic reviews, with more reviews hopefully continuing parallel to the testnet. It is expected that the testnet walk through forks quickly to get to the point where Ring CT transactions are available (in mainnet, it should be half a year, though there was talk about possibly making them available earlier).