I can think of a few ways off-hand.

1) Buy a Matrox RT.X100 real time render card as it does this beautifully and in juicy real time. ok just kidding.

2) Create a seperate title for each letter appearing. i.e. title 1 says "H", 2 says "He", 3, "Hel" untill you get to one that says "Hello World" and then just lay then all next to eachother on the timeline; or

3) Create one title saying "Hello World" and then just keyframe a mask or crop to it.

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I'm not young enough to know everything!