Diablo 3, one purchase for Mac *and* Windows, or one for each?

Today, Diablo 3 went open beta for the weekend, which means I'm finally able to test it on my iMac. I know Blizzard put a lot of work into both platforms so I don't expect them to behave all that different in terms of performance, but I still want to test both.

But a question popped up, and I can't seem to find an answer to this anywhere, perhaps it is implied.

The pre-purchase, or the actual purchase when that comes around, will that be "access to the game" or "access to the game on Windows", and then a separate purchase for Mac?

Basically, when I purchase the game, do I have to pick my platform or do I just purchase access and can switch back and forth at my own whim?

Does anyone know?

If you log in to the Battle.Net site and choose to Download Games, you can download either Windows or Mac client from there. So it would be purchase once and you can use it on either platform.

Blizzard is unique in its cross-platform love: it has bundled Windows and Mac versions together on hybrid discs for over a decade. Your retail or direct download purchase lets you play on either platform. When you buy D3, you're really paying for the Battle.net account CD key. Once it's linked, you can re-download Mac and Windows clients any time you want, but your key will only work on one concurrent installation.