Macs by hardware design can't be upgraded easily using PC parts. They use a newer technology called EFI which provides several improvements including removing the 16-Bit legacy sub system. Despite what people say, Apple appears to actually use some advantages of EFI.
By hardware design you are correct - obviously you can't squeeze a big dedicated card into a machine with no space, little ventilation, small psu and no (official) driver support.
EFI support is in windows and all modern pc hardware, what does apples implementation offer as an advantage (appears?)? It's certainly not speed/stability/security from what I've seen.
That'll happen when all hell freezes over and Linux devs realize GTK doesn't make nice looking GUIs.
You read about valve switching steam off IE to webkit a fair while ago? What about valve hiring linux devs (not saying it's not for their server source obviously)? What about the 'rumours' the steam/source is coming to linux too? Looks pretty much like many articles that I've read before, no characters with penguins on their chest mind. What does GTK have to do with anything?
Using steam as a game delivery service on alternative platforms is good news, will have to see what's available on it in the future. I don't play games, but aren't many of the source engined and *spangly graphics* games fps shooters nowadays? Even as a casual gamer, I wouldn't find osx' mouse acceleration too much fun