You are underinformed my friend. The significance here has nothing to do with the announcement of in-app purchases. The significance is that the old rule was that apps could only do in-app purchases if that app itself costed money. But, obviously driven by market demand/developer pressure, apple has relaxed that policy so free apps can do in-app purchases now too.
That means a free ebook reader like B&N reader, or Kindle, can have in-app purchases managed by the iTunes store. This is, honestly, more notable for helping eliminate the standby Lite vs. Full versions of apps but could easily be used to improve the experience of buying eBooks for reading on an iPhone.
That said, I don't see how this has anything to do whatsoever with the tablet rumors either. But the Apple Table subculture will obsess over the remotest hint of its existence. Just get used to it