How to install iOS 5.1 Simulator if I’m not an iOS Developer Program member?
See Xcode 4.5 and iOS 5.1 Simulator for the solution to this problem:


  I headed over to the downloads page for Apple Developers to get the previous version of Xcode (which is 4.4.1). Downloaded the .dmg and mounted it. Right-clicked the Xcode.app to “Show Package Contents.”
  
  I quickly found what I was looking for in Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs and copied iPhoneSimulator5.1.sdk to the same location in my /Applications/Xcode.app.


The full blog post has some more info.