At least, that's what Brian Ashcraft at Kotaku says about the pre-Tokyo Games Show press event held by Microsoft, where they did show some Halo:

Now, they are showing a Halo 3 trailer. Because, you know, the Japanese really love Halo. It's a new trailer! They showed the Japanese TV ad, too. The Halo 3 Xbox 360 will be open priced when the game goes on sale here.

Kotaku has the story that Yahoo will be hosting two new exclusive Halo 3 videos, Museum and Believe, this week.

Both will debut at 9 PM Pacific Time; Museum on Wednesday, and Believe on Friday. Looks like live action and commercial footage, rather than in-engine cutscene or gameplay.