Now that we’re all using XCode 3.2 on Snow Leopard (you are, right?) and building 64-bit apps you may find that not everything 64-bit works when your app is run on Leopard.

Loren Brichter of Tweetie fame found that Tweeite launched much more slowly on Leopard and I found that 32-bit web plugins (read Flash) do not load in a WebView in a 64-bit app in Leopard.

There’s an Apple approved and supported method for limiting architectures to Mac OS X versions. Here’s how to limit 32/64-bit Intel apps to only run 64-bit Intel on 10.6.0 and later.

Open your application’s Info.plist in a text editor and add the following before the final </dict>

LSMinimumSystemVersionByArchitecture

    x86_64
    10.6.0

That’s it!

Docs for LSMinimumSystemVersionByArchitecture