$('#div1').css('zIndex', 2);

(typeof 2 is number) throws an error in IE7 while

$('#div1').css('zIndex', '2');

(typeof 2 is string) does work.

The error message is "Type conflict" (or something similar, my IE is german), line 544 in jQuery which is around

name = name.replace(/-([a-z])/ig,function(z,b){return b.toUpperCase();});
if ( value != undefined ) elem[name] = value;
return elem[name];