what exactly is involved in enabling bitcode?

You don't even need to explicitly "turn it on" -- the default is on. But yes, it won't work unless all frameworks were compiled with bitcode; so if you are using a framework that wasn't compiled with bitcode, you will have to turn bitcode off in your app.

My understanding is that it is the default, when creating new projects. I would assume that exisiting projects will need to toggle the appropriate setting to actually use bitcode.

It's the default for all iOS projects.

Ok, thanks guys. Sounds like the consensus is that we just have to turn on the setting, and then Apple will do the rest, and that there's no other code changes or configuration required.

Thanks

Surely that's an error if it produces warnings for an optional feature?

It means that all 3rd-party libraries need to be re-sourced and very few library developers will be able to provide those libraries at this stage.

This leaves the developer scratching around trying to find how to disable bitcode.

I've raise bug report 21416003 about this feature.

It's not an error. This is not the first time that upgrading to a newer version of Xcode might require the user to read about the changes and make minor adjustments to make an old project work. You are free to continue using the old version of Xcode if you want.