I'm using XCode 4.4 (recently upgraded from 4.2). Just got my individual iOS developer program license set up today and managed to get my app provisioned and running on a physical iPad. Now I'd like to be able to switch back to the simulator (IOW be able to choose to either run on hardware or on the simulator).
In the top left theres a drop down which will say something like 'iPhone 5.1 Simulator', 'iPad 5.1 Simulator' and 'My iPhone'.
Just choose what you want to run your app on and press play :)
(You can also install other versions of the simulator from the bottom option in the menu which is useful to test your apps)