While you can't currently change your Apple ID to a Mobile/iCloud email address, I found a solution that helps with a related problem.
Suppose you've recently got an iCloud email address, but Apple are still sending all your iTunes Store, App Store and Mac App Store receipts to your old gmail address (the one that's still your Apple ID). And suppose they're also sending all the Apple Support Communities notifcation emails to your old gmail address. How do you change it? How do you fix it?
Well the option exists, but it is hidden away in a weird place. To get Apple sending all of their receipts and notifications to your new iCloud email address...
• Go to your country's Apple Store website http://store.apple.com/
• Click on the Account button in the top-right corner
• Under Account Settings, click on Change your default shipping or billing info
• Sign in with your usual Apple ID and password
• The email address listed under Payment Information is the key
• Click on Edit next to Payment Information and change that email address to your iCloud address
That's all you have to do.