Not sure you can run iTunes store downloaded software through the simulator.
And even if there was an easy way to run iPhone apps on a mac, not sure what the point is?
Many apps are designed around the iPhone/iPod features (GPS, motion sensors) or have limited functionality (e.g. they're for a mobile platform so they do one thing but do it well)... if you think the apps are that awesome the only way to experience them is on an iPhone or iPod touch. They wouldn't be nearly as cool through a simulator.