I am using this in a project,
lblDetail.text =[NSString stringWithFormat:@"%@:%@",lblName.text,lblDetail.text];
If you need a UILabel that can display some Rich-Text, like a text with the first part in blue and the other in red, you can use my OHAttributedLabel class, which is a subclass of UILabel that uses CoreText to draw NSAttributedStrings. (iOS 3.2+)
It is very simple to use and also provide a category for NSAttribtedString that adds some useful methods, like easily change the style of a range of text without the need to know the CoreText attributes constants.
[EDIT] Sorry I didn't realized that Thomas Wanner already mentioned my class in his answer