Does the leader-board in Modern Warfare 2 include players of all platforms?
No it does not. The leader boards are seperate. I imagine nearly all games are like this, as otherwise, you could have a PS3 user with the PSN name: SomeGuy and a Xbox user with the Xbox Live user name SomeGuy, and one may be in the high scores, yet the other could try claim credit for it (or vice versa). Another problem is that it is hard to have any interoperability between Xbox Live and PSN, mainly because Sony and Microsoft don't want it.
No: 360 and PS3 leader boards are separate. The PC doesn't have leader boards.