Just use the newest Application Loader to submit apps. For Snow Leopard Mac OS, you can download Application Loader 2.5 from here to see what the problem really is. If nothing else is wrong, you may get this error:
This bundle is invalid. Apple is not currently accepting applications built with this version of the SDK, xcode.
I am also working on submitting apps with Xcode 4.2, because my macbook is too old to upgrade to Lion and to install Xcode 4.6. It seems that Xcode 4.2 is too old to be supported and the best solution is to install Xcode 4.6.