If you set the frame to allow more than one line along with setting the numberOfLines property this is easily possible. If you plan to put more than one UIlabel on top of one another, for example in a video description page. Here's a nice guide on making a function that can determine the correct UILabel height for a fixed width, # of lines and font size & face:
http://idevkit.com/forums/showthread.php?t=9
According to the docs, sizeToFit _should_ take into account the numberOfLines property, but it simply does not.
Max