
Halo 3 Recon was announced just under a week ago at TGS, however we won't see it on retail stores for another year! Recon will be a stand-alone campaign expansion to Halo 3 detailing the progress of a ODST (Orbital Drop Shock Trooper) who is more or less a soldier who relies on surveying the situation around him and picking his battles carefully - stealth will be key. You won't be the unstoppable Master Chief, but how different the ODST will be is yet to be known. Multiplayer will be the same as Halo 3 as well but with new maps.
To find out some more answers to your possible questions, pleaser refer to last week's Weekly Update from Bungie: http://www.bungie.net/News/content.aspx?type=topnews&cid=15806