Wii Backwards Compatibility is HARDWARE BC; that is, it actually HAS a Gamecube, effectively, there to run it. That's not "totally" true but it's effectively the same.
The 360 uses Software BC; it doesn't have the hardware to exactly replicate an Xbox so it uses Software to emulate the Xbox and run the game. This requires software updates, which Microsoft occasionally (though pretty rarely) releases an update the 360 for more games that it can play. They'll keep this up I assume since they DID add the ability to purchase and download Xbox titles.
Edit: 360 BC list: http://www.xbox.com/en-US/games/backwar ... eslist.htm
Now, the PS3 becomes weird:
Launch PS3s had the Emotion Engine, which provided Hardware BC. However, they eventually stopped this since it cost WAAAY too much for their low sales to warrant it and went the 360 approach of software BC.
Now, as far as I know, the PS3s are probably still "software" BC ... but whatever is already done for that is it. They're not doing more titles than what it has. It could be that these are just lacking ANY BC what-so-ever but they did say Europe PS3 functionality, which had software BC I believe.
Edit: PS3's Backward Compatibility list: http://faq.eu.playstation.com/bc/bcGames.htm
Edit: Holy crap on MGS4 Bundle PS3s (they have BC!) being stupidly expensive ...
