Are you asking from a visual perspective or a user perspective? The user perspective has been tested many times, left aligned labels above fields have consistently won out.

http://www.lukew.com/ff/entry.asp?504http://www.nngroup.com/articles/form-design-white-space/

I've found personally on forms with only 2-3 fields well known fields on mobile you can place the label as the placeholder when space is a consideration. Sign up with email, username, password is a good example. This pattern is becoming well known to mobile app users. Any more fields than that it becomes less effective.