Xbox.com's weekly newsletter points out an interview with Frankie about Halo 3. He says the focus in Halo 3 is playing it your way:

I hope that the main thing players take away is how much Halo 3 is their game. That is to say, it's a game that lets them play it the way they want to, whether that means joining three friends for co-op, or hooking up with eight friends to build a new Multiplayer variant in Forge, or simply share screenshots and movies with their own subset of the Xbox LIVE(R) community. We really have gone all out to make sure this game has more initial impact, and way more replay value than any other Bungie title to date.

There's also something about looking for two (or is it three?) hidden screenshots that littlebigman pointed out in the HBO forum.

Major Nelson posts a note that the Halo Wars "First Look" trailer, clocking in at a tad under 500 Mb in High Definition trim, is now available on Xbox Live Marketplace:

Based on the Halo universe, Halo Wars introduces a new landmark in the strategy genre. Halo Wars places the player in command of human UNSC armies as they deploy for mankind’s initial deadly encounters with the Covenant Empire.

Comments indicate this is the same video that Ensemble has on their website, just in higher quality.

Kotaku has a video it calls a "guided walk" through the gameplay of Halo Wars. No word yet on whether this is the same content as the rumored XBLM video "demo" due out.

UPDATE: Ensemble Studios has put up a ten minute video on their own site; this is what they showed at E3 this year.