The mobilesdk/osx/iphone/builder.py script has a hardcoded value for the minimum iOS deployment target (IPHONEOS_DEPLOYMENT_TARGET) set to "4.0". This is a reasonable default, but there are times when a developer needs to specify a higher version. For example, in my own case, I use a module that requires iOS 5.0 and above, so my application needs to be 5.0+ or I get linker errors.
The workaround is to open the app's project file in XCode and change the setting. This is not ideal for less-experienced developers, especially since it has to be done each time the build directory is removed. It would be better if we could specify the minimum OS version in tiapp.xml.