TBH, this isn't such a big deal; EDC was already halfway to this point. They first decided to pull back from dealing with Amazon in 2009, and since that time the only way EDC titles showed up on Amazon was via a wholesaler. Really, today's move is only an expansion upon previous policy, not an earth shattering. The only real change today is that EDC's wholesaler partners won't be allowed to supply Amazon either.
So EDC won't be losing much from this; in fact, online sales made up only a small percentage of revenues in 2011 (13%).
On the other hand, this could be the first of many publishers breaking ties with Amazon. These things tend to snowball. Do you remember how first Barnes & Noble and then Books-a-Million, Indigo Books & Music, and IndieCommerce decided not to carry any Amazon published titles in store? That took less than a week.
We could see a similar response here. I half expect to see one of the major publishers follow suit this week. They'll do the math and realize that they can take the hit. And I expect it to happen this week.
This assumes, though, that the publisher relies as little on Amazon as EDC does (did). EDC had already survived breaking up with Amazon in 2009, so finalizing the separation was minimally painful.
In any case, 2012 will be a very interesting year.