When Apple announced that it would start selling the iPhone 3G for $99 at this year’s WWDC conference, the room buzzed about the possibility. Finally, there would really be an iPhone for under $100. And so not surprisingly, everyone wants to know how it’s selling. But don’t bother asking Apple about it, because you’ll get nothing from them, as we learned on the company’s earnings conference call today.

Multiple questions were asked of Apple COO Tim Cook about how the $99 iPhone, specifically, was selling. His response? The vague note that the entire iPhone line has seen a “big acceleration in total unit sales.” Cook said Apple will not offer a breakdown of how the different versions are selling because it’s competitive data.

But wait, they’ve broken them up in the past. Just three days after the iPhone 3GS launch, Apple announced that iPhone 3GS-specific sales had hit a million. Obviously, there’s a reason to do that; it’s an impressive number. But along those lines, you can’t help but wonder if Apple is being coy simply because the $99 sales aren’t very good? Certainly, it seems like a great deal, but in reality, the monthly bills of the iPhone 3G and 3GS are the same, so it’s basically the difference between $100 to $200 in upfront costs.

As the call went on today, more questions came in trying to penetrate the $99 iPhone sales topic. Someone asked if the lower price-point iPhone was helping to get people into store and accelerating sales of the new iPhone 3GS? Again, Cook would only say that they’re focused on total iPhone numbers. “It’s too early to tell what the ultimate mix of what the products will be,” he noted. He went on to say that the breakdowns are a big foggy since the 3GS supply is currently constrained, and it’s only in 18 of the nearly 80 countries the iPhone is in.

That would seem to suggest that the $99 iPhone should be selling really well — if customers can’t get an iPhone 3GS. But Cook, nor anyone else would say that. Odd.

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