DVD only, I believe. There's about 20 minutes of added-in footage, with Leon teaching Mathilda to be a "hit girl." There's a scene where the two go into a crack lab (Mathilda has a paintball pistol to practice with) and take out the guy running it that is priceless. Plus a lot of footage that adds more resonance to the end.
When Luc Besson originally wrote it he meant for Mathilda to be about 14, but Natalie Portman so impressed him that he rewrote it to make her younger.