xcodebuild skipping CodeSign step

I have a project that will build fine from the XCode v4.0.2 gui. However, when I run it from the command 开发者_如何学JAVAline via xcodebuild:

xcodebuild -target MyProj -configuration Release -sdk iphoneos

it will run up until ProcessProductPackaging:

    CompileC ...
    CompileC ...
    Ld ...
    Ld ...
    CreateUniversalBinary ...
    CreateDSYMFile ...
    ProcessProductPackaging ...

** BUILD SUCCEEDED **

In the XCode gui I can see it do these steps, but it also does the CodeSign step. Why is it being skipped when I run it from the command line? Isn't xcodebuild supposed to do the same thing as the gui?