What 30-day window? 30-day window of launch day buyers? 30-day window of second-day buyers? 30-day window of last week's buyers? Apple wouldn't place any discrepancy on iPhone buyers depending on when they bought it.
There's no reason Apple would care even the slightest bit about people swapping black iPhone 4s for whites. Here's why.
1) a return with intent to buy is still a sold iPhone for Apple. they don't lose any profit.
2) with black iPhone 4s being returned, more black iPhone 4s will be available to those who want black. We all know that's a lot of people.
3) if you're honestly judging the amount of black-to-white returns by what you see here on MacRumors, remember that all 3 million people who have bought iPhone 4 aren't on MacRumors, and that even still, the amount of people who are saying they will be swapping (on MacRumors) is very little -- at maximum, probably 50. But for argument's sake, let's say that 100,000 people are going to swap black-to-white. I figure that is a reasonable guess. That's still only 0.03% of phones that will be swapped. And still, why would Apple care? See #1.
The "30-day window" has absolutely nothing to do with when the white iPhone 4 will be released. If it happens to be after the "30-day window" (which is still an amazingly vague concept) it'll be mere coincidence. I'm just hoping the release is tomorrow!