Considering the actual structure of the Steam platform and the fact that this wouldn't not be a good marketing strategy overall.
By structure i mean that already any account can redownload every game unlimited time. (I can even download a Valve game i bought in CD-Rom in a store). They clearly want to make steam purchases the more convenient they can be against a regular in-store physical purchase. This probably will apply to steam account on Mac.
As a marketing strategy, the fact that if you buy a Mac game you are "stuck" to play it on a mac forever is one of the biggest factor that stop potential customer from buying games for Mac OS X. BootCamp is a better option right now because it just don't bind you to the Mac forever. By opposition, the biggest advantage with Steam is the fact that you could buy a game and be able to download it for both platform. That's what make Steam potentially a success on OS X. I think they know that, and thats why i think they will use the same account for both platform.
(sorry about my english...

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