Destructoid attended a TGS conference where it was made clear that the game takes place "during the events leading up to Master Chief's return to New Mombasa," meaning that the game takes place between the last two Halo games. The official launch trailer shows an Earth setting, and Bungie's Luke Smith echoed that fact: Recon is "less of a galactic romp" than the original Halo 3.
What isn't so clear is how gameplay might change. Promotional writing hinted that the game might take a stealthy approach, but that made Smith post on NeoGAF that the new game "isn't a stealth action game like a Splinter Cell game." Moreover, playing as a military unit different from Master Chief could mean that core gameplay mechanics like health regeneration could possibly be out the window. "No, I'm not talking about how it's different," teased Smith.
At the very least, Recon will take advantage of Halo 3's multiplayer and community innovations including saved films and the Forge editor. MTV Multiplayer learned from a Microsoft spokesperson that "there are several bonuses and surprises to be found in Halo 3: Recon for those who played and completed the original game."
Perhaps the biggest question of all--whether Recon spells "the end" for Bungie's work on its own baby--is still unanswered. "When asked if that meant Halo 3 Recon would be a 'final statement' from Bungie as far as the Halo trilogy was concerned, [Bungie PR director Brian] Jarrard agreed," reads an interview on GamesIndustry. But, as that only speaks for the Halo trilogy, there still exists the possibility that other Halo-related products could come from Bungie.