I wonder if this means something for Steam? The UX for the Mac App Store is almost certainly going to be better than Steam's clunky app... And then I wonder about Game Centre too? It's not that great at the moment, but with Jobs meeting Zuckerberg the other day....
Probably means nothing for Steam. Steam has the big advantage of being cross-platform. Game devs planning to only target the Mac (or Mac and iPhone/iPad) would probably be better off going with the App Store.
Developers planning to go cross-platform to Windows would be better off with Steam.
Actually, the app store for ipad is pretty damn clunky (steam isn't great, but I like it better). Honestly, I dont think they have anything to fear... They are, after all, cross platform. Game Center is a pile of schlock at the moment... I suspect, at least for AAA titles... steam will remain top dog.
But yes - definitely excited!
/me stops speculating and gets back to work