Let me start by saying that it should not have taken nearly a decade for Canon to get around to doing this, especially when it appears that they did it by accident.
I know that Michael has ripped on Canon for not including mirror lock up button in their DSLRs. I have to agree with him, Canon has stubbornly, some might even argue stupidly, failed to implement a half way decent mirror lock up functionality in any of their DSLR*. Well I think with the 5DII Canon has finally implemented this, and guess what, they even gave mirror lock up it's own button. Now you might be wondering what in the world I'm talking about since when you look at any of the sites that cover the various buttons and features of this camera they don't mention it. What this is that the Live View implementation on this camera doesn't flip the mirror down prior to the shot. So you can frame, focus, flip the mirror up, and then shoot just as if MLU had it's own button.
*I had an old Elan II that had a very decent mirror lock up function where you could turn on mirror lockup up on whenever the camera was in 2 second self timer mode. I always felt like this was a very reasonable way to do this and why I separate the DSLRs from the SLRs.