If I'm on a diet can I drink chai lattes with soy milk ??!?

You can, but don't go ginormous with the sizes. There's a lot of sugar in chai, no matter what kind of milk you use. So stick with the tall/small size and you should be fine. Think of it as a treat or a dessert instead of a 'drink'. That way, you won't fool yourself into thinking it has less calories than it actually does.