You need to add:
txtField.textAlignment = NSTextAlignmentCenter; // Pre-iOS6 SDK: UITextAlignmentCenter
Keep in mind, this adjusts both the alignment of the placeholder text as well as the text the user will enter in.
How to center vertically
Since the original question was updated to request how to vertically align the placeholder text and that answer is buried in the comments, here is how to do that:
txtField.contentVerticalAlignment = UIControlContentVerticalAlignmentCenter;