$('selector').css('line-height');
Returns line height as a decimal value in pixels, with unit, e.g. "22.5px"
Care should be taken with this solution, as different browsers will report different results.
I have found that you cannot use unitless measures with line-height (eg 2.2) in your CSS, as IE6 will report an incorrect result (eg 2px instead of, eg 36px).
So I recommend using unit-based line-heights with this measure.
