Thanks for the heads up on this thread.
Upon subscribing to the feed, the iTunes application should request a username/password. Subsequent requests in the same iTunes session should not require additional authorization. However, if the user is already subscribed, and returns for a later session, they might be prompted again to enter authorization details before being allowed to access any protected media made available in the feed. Authorization is good for one session of iTunes in most cases, depending upon the configuration of iTunes on the user’s computer.
Please see this video tutorial: http://www.s2member.com/videos/71F49478D6983A9C/
is Apple ok with having paid content in the store?
I’m not aware of any policies that iTunes may have about this within the store. I’m aware that the iTunes application does support protected feeds and content, but whether they allow it to be listed in the store (or not), I’m not sure about. Please let us know if you have an experience you can share.