According to the Wall Street Journal, Apple will require any subscription-based periodical apps on the App Store to offer an in-app purchase option by March 31 or else face rejection. While that may sound like a fairly draconian demand on Apple's part, the Journal also notes (albeit way down in the article) that Apple is still allowing publishers to offer subscription purchasing options outside the App Store so long as in-app purchasing is offered, too.
While the Journal says Apple hasn't commented on what this means for e-book apps like Amazon's Kindle reader, I'll go out on a limb and say that Sony's recent "the sky is falling" media frenzy following the rejection of its Reader app was likely much ado about nothing. So long as Apple still allows content purchased outside of the app's in-app purchase mechanism, there doesn't seem to be anything untoward going on here.
The road ahead still looks slightly bumpy, however; while in-app subscription purchasing is a potential boon to users, there's no indication that Apple's giving up its 30 percent cut of in-app purchase sales. That might be more than some companies are willing to swallow, Amazon in particular, so it's unclear how certain publishers will react to the mandatory in-app purchasing rules even if off-app purchasing is still allowed. We'll be watching very carefully how Apple's relationship with publishers plays out over the next couple of months