The Xbox 360 has to emulate old Xbox games. No, not all of the old titles will play on it at this time, but I'd say this list has considerably more than Halo 1 and 2 on it. With Xbox Live's marketplace feature, it should be easy for Microsoft to add updates for each old game; I would be surprised if, within a few months, there are more than a handful of old titles that are still incompatible. And sure, Sony says the PS3 will be completely backwards compatible, but that doesn't neccessarily mean it will happen.
* AirForce Delta Storm
* Alias™
* Aliens versus Predator™ Extinction
* All-Star Baseball™ 2003
* Amped®: Freestyle Snowboarding
* Army Men®: Sarge's War™
* Atari® Anthology™
* ATV: Quad Power Racing 2™
* Baldur's Gate™: Dark Alliance™ II
* Barbarian
* Barbie™ Horse Adventures™ Wild Horse Rescue™
* Batman Begins™
* Battle Engine Aquila
* Battlestar Galactica™
* BMX XXX™
* Brute Force™
* Buffy the Vampire Slayer™: Chaos Bleeds
* Cabela’s® Dangerous Hunts™
* Cabela’s® Outdoor Adventures™ 06
* Cabela's® Deer Hunt™ 2005 Season
* Cabela's® Deer Hunt™ 2004 Season
* Call of Cthulhu®: Dark Corners of the Earth™
* Call of Duty™: Finest Hour™
* Casino
* Chicago Enforcer
* Circus Maximus
* Close Combat: First to Fight
* Colin McRae Rally 4
* Combat Elite: WWII Paratroopers™
* Commandos 2: Men of Courage™
* Conflict: Desert Storm
* Constantine™
* Crash Twinsanity™
* Crash Nitro Kart™**
* Crimson Skies®: High Road to Revenge™
* Crouching Tiger, Hidden Dragon™
* Dark Angel
* Darkwatch™
* Dead or Alive® 3
* Deathrow
* Digimon® Rumble Arena®
* Dinotopia**
* Drake™
* Egg Mania™: Eggstreme Madness
* ESPN MLS ExtraTime 2002
* Euro 2004
* F1 2001
* Fable®
* Fable®: The Lost Chapters™
* Fairly Odd Parents: Breakin' da Rules
* FIFA Soccer 2003
* FIFA Soccer 2004
* FIFA STREET
* Fight Night 2004
* Fight Night Round 2
* Ford Mustang
* Ford vs. Chevy
* Forza Motorsport™
* Frogger Beyond™
* Futurama™
* Fuzion Frenzy®
* Genma Onimusha
* Goblin Commander™: Unleash the Horde
* Grand Theft Auto 3
* Grand Theft Auto: Vice City
* Grand Theft Auto: San Andreas™
* Gravity Games Bike: Street. Vert. Dirt.
* Grooverider: Slot Car Thunder
* Half-Life® 2
* Halo®
* Halo® 2
* Halo® 2 Multiplayer Map Pack
* Harry Potter and the Goblet of Fire™
* Harry Potter and the Sorcerer's Stone™
* He-Man: Defender of Grayskull
* Hitman: Contracts™
* House of the Dead 3
* IHRA Drag Racing Sportsman Edition
* IHRA Professional Drag Racing 2005™
* Jade Empire™
* James Bond 007™: NightFire™
* Judge Dredd®: Dredd vs. Death™
* Jurassic Park: Operation Genesis
* Kabuki™ Warriors
* Kelly Slater's Pro Surfer
* kill.switch
* Lemony Snicket's A Series of Unfortunate Events™
* LOONS—The Fight for Fame
* Manhunt
* Mat Hoffman's Pro BMX™ 2
* Max Payne
* Max Payne® 2
* Medal of Honor European Assault™
* Medal of Honor™ Frontline
* Medal of Honor™ Rising Sun
* Mega Man® Anniversary Collection
* Metal Arms™: Glitch in the System
* MicroMachines
* Mike Tyson Heavyweight Boxing
* Monster Garage™
* Mortal Kombat®: Deception™
* MTV Music Generator™ 3
* Murakumo: Renegade Mech Pursuit
* MX World Tour™: Featuring Jamie Little
* Namco Museum™
* NBA LIVE 2004
* Need For Speed™ Underground 2
* NFL Blitz 2002
* NFL Blitz 2003
* NFL Blitz 2004
* NHL® 2004
* NHL Hitz™ 2003
* Ninja Gaiden®
* Ninja Gaiden® Black
* Outlaw Golf 2
* Outlaw Volleyball
* Pariah™
* Phantom Crash™
* Pinball Hall of Fame™
* Pitfall®: The Lost Expedition™
* Predator Concrete Jungle™
* Prince of Persia: The Sands of Time™
* Pro Evolution Soccer 5
* Pro Race Driver™
* Pump It Up™: Exceed
* Pure Pinball
* Puyo Pop Fever**
* Quantum Redshift®
* Rayman Arena
* Raze's Hell™
* Red Dead Revolver
* Red Faction® II
* RedCard™ 2003
* Robotech: Battlecry
* Rocky™ Legends
* Rogue Ops
* Samurai Jack
* Samurai Warriors™
* Scooby Doo!™ Night of 100 Frights
* Scrapland
* SEGA GT™ 2002
* Shadow The Hedgehog™
* Shamu's Deep Sea Adventures
* ShellShock: Nam '67™
* Sneakers™
* Sniper Elite
* Soccer Slam
* Sonic Heroes™
* Sonic Mega Collection™ Plus
* Speed Kings™
* Sphinx and the Cursed Mummy™
* Splat Magazine Renegade Paintball
* SpongeBob SquarePants™: Battle for Bikini Bottom
* SpyHunter® 2
* Spyro™ A Hero's Tail
* SSX 3
* Stake™
* Star Trek: Shattered Universe
* Star Wars®: Episode III Revenge of the Sith™
* Star Wars® Jedi Knight®: Jedi Academy™
* Star Wars: Starfighter™ Special Edition
* Star Wars®: Knights of the Old Republic™
* Star Wars® Knights of the Old Republic® II: The Sith Lords™
* Street Racing Syndicate™
* Stubbs the Zombie™ in Rebel without a Pulse™
* Super Bubble Pop
* Super Monkey Ball™ Deluxe
* SX Superstar™
* Tecmo Classic Arcade
* Teenage Mutant Ninja Turtles™
* Test Drive®: Eve of Destruction
* Tetris Worlds™
* The Great Escape
* The Hulk™
* The Incredible Hulk™: Ultimate Destruction
* The Incredibles: Rise of the Underminer
* The Lord of the Rings™: The Return of the King™
* The Simpsons™ Hit and Run™
* The Simpsons™ Road Rage
* The Terminator™ Dawn of Fate
* The Thing™
* Thief: Deadly Shadows™
* Tom Clancy's Ghost Recon
* Tom Clancy's Rainbow Six® 3
* Tony Hawk's American Wasteland
* Tony Hawk's Pro Skater™ 4
* Tony Hawk's Underground 2
* Tork™: Prehistoric Punk™
* Toxic Grind
* Ty The Tasmanian Tiger™
* Ty the Tasmanian Tiger™ 2: Bush Rescue™
* Ty the Tasmanian Tiger™ 3: Night of the Quinkan
* Urban Freestyle Soccer
* Vexx™
* Volvo: Drive for Life
* World Series® Baseball 2K3
* Worms 4 Mayhem
* Worms™ Forts: Under Siege
* WWE™ Raw™ 2
* XIII
* Yourself!Fitness™