IDE releases always FOLLOW API releases rather than preceding them. Its been my observation,there aren't always new IDE releases with every API release. There often tends be only be 1 or perhaps 2/year. I suspect it depends a great deal on how significant the changes are to the API. In part, this also prevents someone from inadvertently attempting to prematurely implement new API code into production; a minor thing at best since the code would be rejected.
More than anything else, I suspect SF is focused on getting the new API finalized and released, not the IDE which it can't fully do until the API is finalized and "Gold" so to speak. When you pause to think about it, how can you expect them to release an IDE prior to the API being finalized? We'd have a beta IDE for for use with both a beta API and a production API. Not the wisest thing to do.
But I agree with @dphil that SF doesn't appear to be highly invested in supporting Eclipse and leaves the impression they'd prefer we use the Developer Console. Yet, its clear they know that's not realistic for serious developers. Our Dallas DUG received a list from them last fall to help us prepare a presentation on SF's in-house favorite tools their developers use. Clearly, their own developers use more than the Developer Console for their work and to collaborate with one another.