Any number of things could cause faint yellow spots (especially in the early stages), have you been through the tomato problem solver to get ideas yet? For example, early blight might show as faint yellow spotting at first. If you could post a pic, that might help with identification.

I agree, it's probably early here for leaf mold, and the conditions really haven't been right for powdery mildew either. But just because I said that doesn't mean it's completely impossible -- be sure and also look at those pics in the problem solver as well.

Here is a link that might be useful: tomato problem solver