I often need to underline the text of an UILabel in my iOS projects in a specific way. Here's how I've done it.

Create a subclass of UILabel, call it e.g. RGUnderlineText and overridde the
- (void)drawRect:(CGRect)rect method.

- (void)drawRect:(CGRect)rect {
    /* draw a dotted line at the bottom of the UILabel */
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetRGBStrokeColor(context, 1.0f, 0.f, 0.0f, 1.0f);

    CGFloat lengths[] = {1,1};
    CGContextSetLineDash(context, 0, lengths, 2);

    CGContextSetLineWidth(context, 1.0f);

    CGContextMoveToPoint(context, 0, self.bounds.size.height - 4);
    CGContextAddLineToPoint(context, self.bounds.size.width,
                                     self.bounds.size.height - 4);

    CGContextStrokePath(context);

    /* draw the typical UILabel now */
    [super drawRect:rect];  
}

The drawRect code draws a dotted line at the bottom of the UILabel and calls super to get the normal text drawn. You can of course adjust the code/drawing to your needs.

In Interface Builder drag an UILabel element from the Library on a view and set the class to your new subclass of UILabel, e.g. RGUnderlineText in my case. That's it.

The sample code is available as a XCode project at bitbucket.org.

Here's a screenshot of the final result: