The complete description of the new Flexible Raid Lock System is on MMO Champion. I wanted to give you a few thoughts about it because it can change a few things for many players, especially if you only have one main character.
To say things simply, you can now only down an ICC boss once a week, either in 10-Man or 25-Man (I'll address heroic mode later). You can move from a 10-Man to 25-Man raid or the other way as long as the raid you join has down at least all the boss of your previous tag (it can have down more).
For heroics, your tag is mostly fix as it is now, you can not join an heroic tag different from yours, though you can freely move from heroic to normal.
Only problem I can see is for smaller guilds playing mostly 10-Man (as mine). If your guild is running 10-Man with your main char, this character will be locked out of 25-Man even though he could need gear in 25-Man. That system will be perfect once the loot is the same in 10 and 25-Man but it is not yet the case and I can see groups having problems with it until Cataclysm.
I probably have too many alts to have any such problem, but not everyone has more than one 80 ...
what do you think about it ?