Apparently this is rather tricky as it requires use of a Xcode private framework. These links may help:

http://cocoamanifest.net/articles/2011/12/running-your-ios-app-in-the-simulator-from-the-command-line.html
http://cocoamanifest.net/articles/2012/04/xcode-4-3-fix-for-simulating-from-the-command-line.html
https://github.com/jhaynie/iphonesim (a fork of this was used for the upcoming iOS support in Qt Creator 3.0)
https://github.com/square/WaxSim
https://github.com/jonathanpenn/WaxSim
https://github.com/0xced/simlaunch