:hover pseudo selectors only work on links in IE6. You have to make it work on non-link elements, too. What you do? Your first thought is IE's magic equipment, Mr. Expression. You'll be trying, but won't succeed. But project deadline is coming, so you go to your template and hardcode onmouseovers and onmouseouts in your tag soup. Oh no, you say. Oh no. You cry. Later on you periodically review your code, and your eyes always stop on these onmouseovers and onmouseouts, and you always cry.
You see? We check if the assigment has already done and hence it will be blazing fast. That's the big idea.
_m: expression(onmouseover = onmouseover || new Function("$(this).down().style.display = 'block'") : '');
_n: expression(onmouseout = onmouseout || new Function("$(this).down().style.display = 'none'") : '');
Essentially that's all that I wanted to share with you, my precious fellows. Two more notes: String assigment to the onmouseover and onmouseout properties won't work. You have to create functions explicitely. Lastly, no need for "this." prefix, however this is the nice way, but remember, in hacks there's no nice way.
Fine, an even more beautiful snippet is here.