I've been trying to figure out how to get a multi-line label working with wordwrap and such in MonoTouch.  The code below will help out.  The key thing that I found is that you HAVE TO set the value of the UILabel BEFORE you call the .SizeToFit().

        public string TwtStatus{
            get{ return _TwitterStatus; }
            set{
                _TwitterStatus = value;
                TwitterStatus.Text = _TwitterStatus;
                TwitterStatus.Lines = 0;
                TwitterStatus.LineBreakMode = UILineBreakMode.WordWrap;
                TwitterStatus.Font = UIFont.FromName("Helvetica", 12);
                TwitterStatus.SizeToFit();
            }
        }

 IIRC, I snagged this code from a blog post somewhere.  Of course, I was the one that broke it by setting .Text to a value after I called the .SizeToFit() method.