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Beginning with the September issue, existing U.S. print subscribers can get six months of free access to the magazine's iPad edition [iTunes link] by entering their credentials.
After that, they'll need to sign up for digital subscription, available for $1.99 per month or $14.99 per year. (Non-print subscribers can also get the digital edition at that price.) Single issues are available for $1.99 apiece.
Reader's Digest published an experimental version of its February issue for the iPad earlier this year, which was pretty decent as far as iPad versions go -- nicely formatted with a few bonus games and polls, etc. This, however, marks the beginning of a regular publishing cycle on the device.