It will be hacked on the 360, they have a brilliant community.
Not on PS3 though, we hardly have anything or people that are willing to release.
Quite sad really but thats the truth.

a game getting hacked doesnt make it a fail...

mw2 had the 10th prestige hack, but did the game fail?? no, so many people continued to play it.
The COD4 UFO hack, people continued to play it, it still gets played now, 3 years down the line and its still being played. Not a fail.
Black ops, the game fails for the people who got caught hacking it, everyone else continued to play it, thus another game not being a fail.
Finally World at War had some many glitches, elevators and wall breaches, people stilled played it (i personally didnt like the game so went back to COD4)

Modern Warfare 3, nothing has been released about its security or anything as of yet. but tbh, every COD so far have been exposed, but not ruined. So my opinion is that the game will not fail down to hacking, it will most likely be successful for years to come

I think it will be harder to mod than MW2, but easier to mod than blackops. Most of the work that IW do is on Multiplayer features and story line. They also like to work a lot on the balance of the game. They tend to work on the security the day before they release their game.