Speaking to Joystiq, Bungie's Brian Jarrard has said that Halo 3: Recon should be seen as two products.
"Think of Halo 3: Recon really as two games in one," he said. "You have the new Recon campaign that is a separate 'title ID' and includes 1,000 GamerScore and Achievements, etc. It's campaign only.
"The multiplayer maps are more Halo 3 goodness, still considered 'regular' Halo 3 content and the multiplayer stuff is interchangeable in that regard. If you purchase the Mythic Map pack via the Xbox Live Marketplace early next year, but your friend doesn't get those maps until the Recon disc comes out, they will work together just fine."
The game releases in spring 2009. Plenty more through there.