(Pfew, I found my WordPress account again!)
sIFR actually checks for element.offsetHeight
, and removes the vertical padding from that number. That way you can get near the line-height almost perfectly in virtually all browsers. You have to specify the padding though.
You can also use getComputedStyle and currentStyle to get at line-height directly (consult Google for more on that).
If you have any further questions, please mail me (https://novemberborn.net/colophon/contact), as I’m not checking these forums anymore (and I’m not at home now, so I tend to forget about things like this)).