http://www.t3.com/feature/halo-3-mythic-map-pack-exclusive-bungie-interview

T3: If I have a copy of ODST and my friend buys the Mythic Map Pack, will we be able to play together?

Lars Bakken: Yes, absolutely. You’ll be able to play Halo 3 multiplayer together. There is no difference there.

T3: Apart from map content, will there be any other kind of crossover – Master Chief versus ODST deathmatches?

Lars Bakken:  Hahahaha... Now, that’s a cool idea. Nothing like that though. ODST is its own story, and we’re trying to keep them separate as this is a side story.

T3: So, to get this straight - to get the Mythic map pack, you can buy the special Halo Wars version, the standalone map pack or Halo 3 ODST – could that be confusing?

Brian Jarrard:  Halo 3: ODST will include the complete Halo 3 multiplayer experience which does mean that a lot of players may already have purchased some of the DLC maps over the past year or two. However, there are some brand new exclusive maps and with so much other great content and the ODST campaign also being stuffed into the box, it’s our hope that the final price reflects a great value and doesn’t give anyone the impression that they are paying for something they already bought. 

Wait... So ODST players can play with Halo 3 PLayers.... I guess this means no new online weapons... :(

Also, in the interview, they said no Master Chief vs ODSTs.

Yeah, but then they're talking about the player meeting Master Chief in the game...

I think rebalancing the multiplayer so that the Master Chief could fairly fight any group of normal soldiers, no matter how elite, would be almost impossible.

wait so u get extra maps plus the dlc maps?

There are 6 unreleased maps going from the new Halo 3 achievements.  I think it would be safe to assume that since 3 of those shipping with Halo Wars (Mythic Maps) that the other 3 will probably correspond with Halo: ODST

 i already knew that but r they going to have other new maps with it

are they going to gouge by charging 60 bucks for what seems to be more or less an expansion/DLC type thing or are they going to make it 30-40?

It really should be a 30 buck DLC, it doesn't seem THAT much more involved then the new GTA4 DLC is in that it uses the same engine and everything.

It's an entirely new game with a completely new campaign and completely dissimilar combat mechanics with an original narrative. What more do you want?