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I've sent G+ messages to people who have not added me to a circle. This is supported, just like on Facebook.

Good question. I hadn't really thought that far ahead yet, about the most ideal form for a developer to respond to comments - just that there finally will be a method. I suppose, first, that it depends on how Google implements this. There may be comments allowed on the comments left by users, and then the obvious move is for developers to respond there. Otherwise, I suppose the developer could make a G+ post and share it with just the person who commented, or send the user an email if that's an option.

It works like twitter, you type "+username blah blah blah", if you want it to be a completely private message, remove any other people/circles in the share to box.