Yesterday I was doing some coding and I wanted to increase line-height of Text component, and saw that there is no line-height style property in Flex, like there is for example in CSS. So google helped me once again and I wound this article http://blog.flexexamples.com/2008/05/31/setting-the-leading-on-a-text-control-in-flex/.
It states that you use property leading to set line-height (that is the distance from bottom of first row of text to the top of next line of text).
If I have multiline text and want to increase leading or line-height, I can just do the following:
<mx:Text text="Here is a loooong string that will span multiple lines" leading="5" />
Below is an image that shows the difference in Text component if it has leading set to 1 and to 5. (screenshot is taken from blog.flexexamples.com)