This wouldn't happen. Not now. Not ever. The vast difference between Sony's resources and Microsoft's is one reason. There is also the fact that Live is a subsidiary of Microsoft. Microsoft, you know one of the largest corporations on Earth owned by the second richest man in the world that you would have to be brain dead to take on or try to hack. Theres also the fact that people knew that Sony's servers were insecure. You hear all these people saying they knew they were insecure, but you don't hear that about Live. And another reason is that the hated "You have to pay" feature makes it where their are a lot of people dedicated to it and gives it a higher priority within Microsoft than the PS3 has in Sony, because the Xbox and it's services ae a money maker unlike the PS3 which is still losing money on its consoles.
tl;dr Live wont be hacked
Now I wait, because this site has many more PS3 fanboys than Xbox.