With all due respect, but that is not a very smart idea.
You would like to have empty text on password fields as well.
Let us say you have two text fields, username and password. Instead of putting labels, you can just set empty texts in both fields to notify that one is username and the other one is password.
Same way facebook is doing.
I think this behavior should be provided by the library itself... It looks kind of weird if you have empty texts for all your fields EXCEPT the password field.... so if you want to have a nice look everyone has to implement this himself...