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You *can* sign with Developer ID on a Mac running Snow Leopard.  It is not officially supported, but it is unofficially supported and somewhat documented.

You need a Mac running Xcode 4.3+ in order to retrieve your Signing Certificate(s).  You then export this from Xcode 4.3+, then import into Keychain Access in your Build Mac.  Then you can sign anything with Developer ID on your Build Mac using the 'codesign' command-line tool.  I am running Xcode 3.2.5 under 10.7 (not supported, but I do it anyhow), and have successfully added this step to my build script and it works.

Personally, I haven't had any trouble building and signing with Xcode 3.2.5. However, in order to get applications into the App Store successfully you will need to build retina Icons on Lion or Mountain Lion using iconutil and then use these icns in your Xcode 3.2.5 build.

Labels: codesigning, lion, mac app store, mountain lion, retina