1Up's latest piece on Halo 3 covers the "Bungie Recommends" feature that will take game variants and Forge map variants from individual player shares and suggest them to the community at large. That'll be how custom community-made gametypes, like Halo 2's honor-rules zombie, or dawn of the dead variants, will spread in Halo 3. They've also continued their Halo 2 retrospective up through the Arbiter levels. You can see all 1Up's Halo 3 coverage on their Halo 3 mini-site.