Except that you are kinda forced to pay for the iPhone developer portal unless you want to jailbreak your iPhone since you need those certificates and provisioning profiles.<p>But to develop for the Mac you just sign up for free and download XCode (or get it from the install DVD, albeit outdated).<p>The only advantage is probably getting access to pre-released versions of OSX so you could test your app but I somehow doubt there is such a large number of people that need to do this that badly and couldn't afford the previous $500.