Many people see this as Amazon trying to grab marketshare from the traditional publishers. For the most part, I agree. However, I believe that Amazon is announcing this now for another reason: as a pre-emptive strike against the eagerly-anticipated Apple tablet that is expected to be announced next Wednesday. Why else announce a change that won't take effect until June?
The 70/30 split is the exact same formula that Apple uses for apps sold through its iTunes store. The pundits are saying that the tablet will be a direct competitor to the Kindle, and that Apple is in talks with various publishers regarding the device.
Amazon now faces competition from B&N's Nook (sorry fellas - I had the name first!) and the Apple tablet - so it's trying to do everything it can to lure authors to its platform. This competition is great news for eBook authors.
With Amazon's hefty royalty offer, and Apple moving into the space, it should prove to be a very interesting (and profitable?) 2010 for authors that choose a less traditional route to publishing.