Mission of Honor slated for July 2010 release.
You can find a couple minor snippets on the appropriate forum at http://www.davidweber.net. Major snippetting probably won't begin for a couple months yet.
If I've had one issue with recent (well, as recent as can be for a series last whose last novel was--5?--years ago) mainline HH novels it's that the political maneuvering that doesn't directly involve Honor often makes my eyes glaze over, and I find myself skimming it and not paying particularly close attention. Reading the description of this book, however, I suspect I'm going to have to go back and re-read At All Costs and maybe even War of Honor, because I have no clue why the Sollies have their panties in a twist, except that it likely has something to do with that system the Star Kingdom annexed. Of course, the five years of downtime since At All Costs came out may have something to do with that as well.
It also probably wouldn't hurt to finally break down and read the two novels in each of the two offshoot Honorverse series (Shadow of Saganami and Storm from the Shadows, and Crown of Slaves and Torch of Freedom) since I imagine the events in those are relevant in some tangential way to the setup for Mission of Honor.