Seriously, OSX does NOT do that.
A single threaded app CANNOT be spread across multiple cores. Not on /any/ OS.
On both OSX and Windows (and Linux), multithreaded apps (even old ones) can and DO get spread across multiple cores by default. But ancient, single threaded DOS apps (Which require an emulator in Windows 7 now anyway) will only run on one core ever.
A LOT of what Apple is claiming about OSX is simply hyperbol. There is nothing new about hibernation, there is nothing new about fullscreen apps and there is nothing new about multicore use on multithreaded apps. (At least not on windows
Disabling core parking will ususaly give no noticable perfomance increase in reality (though maybe a little in very certain circumstances) but it will increase power use. So definately leave on for laptops. On desktop it won't really change much. I've never even seen it kick in on my desktop myself...