Now that iPad mini has been released, is there anything an UI designer needs to look out for? I understand that the resolution and screen ratio remains the same, and I've been hearing that no change needs to be done for the existing apps. However, everything is now ~80% smaller on screen, which means that the buttons are now smaller. I am especially concerned about the Navigation bar at the top, where the buttons (back/edit/add buttons) were already quite small but will now be even smaller. I can see that as a potential problem for users with big fingers, but increasing the button size means that the Navigation bar height would have to be increased as well, if that's even possible. Is there another way of fixing this? Also, how does the 44x44 point minimum tappable area apply to iPad mini?
According to Microsoft guidelines, comfortable touch targets are 9mm is size, or smaller with at least 9mm in between each. Moderate is 7mm and minimum is 5mm. I would check whether your application follows those guidelines. If the action that the user will have to take is frequent or mission-critical, make the targets 9mm or a close to that as you can.
Windows Phone 7 touch guidelines: http://msdn.microsoft.com/en-us/library/windowsphone/develop/hh202889(v=vs.92).aspx
PS those are backed up by extensive research.
There is nothing special to optimize for, if you stuck to the recommendations for the minimum target sizes in the first place. The targets on the new iPad mini are exactly the same size as they would be on the iPhone 3G(S), as the screen resolution is the same (in pixels per inch). So, you shouldn't have any trouble. If you now have trouble with targets becomming too small, you did not stick to the guidelines in the first place.
Not having seen an iPad mini I can only theorize:
I'd say take the cue from Apple. I assume they've given the mini a good round of user testing and if they haven't changed the pixel dimensions of the buttons on built in apps (iTunes, App Store, Settings, etc.) then it's probably OK. People will adjust to the 20% smaller size, their field of view will be focused tighter on the 20% smaller display and gestures will tend to be slightly more accurate. I think.