How can I make text in a UILabel shrink font size

descriptionLabel.adjustsFontSizeToFitWidth = YES;
descriptionLabel.minimumFontSize = 10.0; //adjust to preference obviously

The following example is tested and verified on iPhone Simulator 3.1.2:

UILabel *descriptionLabel = [[UILabel alloc] initWithFrame:CGRectMake(90, 0, 200, 30)];

descriptionLabel.font = [UIFont systemFontOfSize:14.0];
descriptionLabel.minimumFontSize = 10.0;
descriptionLabel.adjustsFontSizeToFitWidth = YES;
descriptionLabel.numberOfLines = 1;
descriptionLabel.text = @"supercalifragilisticexpialidocious even thought he sound of it is something quite attrocious";