[teyc]

As a hobbyist, you could always use MonoMac. It is still free and you may have to wrap the StoreKit APIs yourself.

[lucaspiller]

+1. Xamarin.Mac just builds more on top of MonoMac. MonoMac has been used to publish apps to the App Store before:

http://tirania.org/monomac/archive/2011/Jan-31.html