In the latest Bungie Weekly Update they revealed a new camera feature called Forge Filters.  This filters allow mapmakers assign image filters to certain areas of the map.  These filters can be experienced by gamers in real-time during multiplayer. 

This means that you can venture to a certain part of the map and all of sudden be playing in Black-and-White mode, or Sepia mode, or Pen and Ink mode.  The filters are Forge objects so I guess there will be a limited number of these to go in each map.

This will add an interesting twist to custom maps and gameplay in Halo 3.  It could be interesting to see what happens when you can't see what color the other team is.

Nova mode

Old Timey mode

Pen and Ink mode

Some more details on Forge, the map editor for HALO 3, come from an IGN report.

• New mode/feature called the “Forge” lets you edit weapon placement, spawn points, vehicles and more in real-time during the middle of multiplayer matches. You’ll take on the shape of monitor, similar to the Guilty Spark, and will even be vulnerable to gunfire and player attacks while you edit maps in real-time.
• Multiplayer customization options let you create your very own power-ups with specific attributes. For example, if you want to create a power-up that lets you run twice as fast, kill in a single shot and jump extra high, you can.
• You can share your edited levels with others by placing it in your “shared items” slot, which other players can then access and download to their 360.
• “Bungie Recommends” section on Bungie.net will spotlight Bungie-found user content, like screens, videos, edited levels, rule sets and more.
• Heroic and Legendary difficulties now tougher than ever. Bungie recommends experienced Halo gamers play on these difficulties rather than Normal.

Hrmm. Making customizable power-ups. My only concern is NOT knowing what power-ups are in the map. I would hate to be running around Valhalla and get beaten by someone because they place their custom power-ups somewhere on the map. But, Forge is going to add at a lot of replay and fun to the game. Forge is going to drive more people to sign up on XBOX Live.

The press has recently attended preview sessions at Bungie HQ.  They've gotten to play parts of the campaign and magazines have been given renderings and screenshots to publish.  HALO 3 looks mature, spectacular, and deeper than HALO 2.  See what I mean:

Scans of Famitsu H3 images from Neogaf.com
Scans of Joypad H3 images at JeuxFrance.com
Summary of Interview with Jamie Griesemer from MSN.ca:

The Scarab conflict is part of The Storm, the fourth mission of the single-player campaign component of "Halo 3," and just one portion of the mission, has more enemies and allies than the entire first mission of "Halo 1."

The size really ramps up as you go through."But in 'Halo 3,' The Scarab is an AI (artificial intelligence). He'll chase you around, he'll acquire targets. If he can't find you, he'll search for you. He's the biggest character we've every done, essentially. And there are dozens of ways to kill him."

"All equipment in multiplayer shows up in single-player and the AI will use it on you," said Griesemer."The main way you're going to get equipment in the single-player game is by killing guys before they can use their own (on you)."

On the multiplayer front, a nifty feature called The Forge allows the gamer to redesign maps by changing the placement of weapons and vehicles, as well as changing the rules of engagement.

The game becomes more complicated via hidden skulls. Scattered through the game, these change the rules - for example, one skull calls for both players to die and be sent back to their last checkpoint if just one player is killed.

The ultimate task will be to finish every mission on legendary difficulty, with all skulls turned on, in a certain time limit with style.

It's looking like HALO 3 will not disappoint on the campaign front.  We are going to get more vehicles, more enemies, better vehicles and better enemies.  We are going to get rich environments and complex engagements that are sure to add dozens, if not hundreds, of hours of replay value to the campaign.