That's incorrect. Apps that run in the iOS Simulator are compiled to i386 code and run as a native OS X processes. They are essentially OS X apps that link to UIKit instead of AppKit.
Edit: That's not to say Apple couldn't support such a scenario. Many App Store apps are already fat binaries that include ARMv6 and ARMv7 code. No reason why they couldn't include i386 code as well to facilitate running on Mac hardware.
Apple has filed a patent on OS-level advertising. http://ipwatchdog.com/2009/10/22/jobs-and-apple-seek-patent-...
Integrating iOS apps (where users are already conditioned to being exposed to advertising) with some version of OS X would not be a big leap from a technical or business strategy perspective.
I wouldn't be a bit surprised if future versions, maybe even the upcoming release, set up an equivalent situation on the mac platform, now that Apple has a new TV device in addition to the iPhone, iPod, and iPad.
Heck, if Apple decided to be aggressive about getting into the living room and entertainment center, they could launch a box that's basically a souped up Apple TV with something close to an order of magnitude higher performance than an XBox 360 simply by building it around a Sandy Bridge, and have a significant contender in the gaming console market.