james edwards ie conditional

// ----------------------------------------------------------
// If you're not in IE (or IE version is less than 5) then:
//     ie === undefined
// If you're in IE (>5) then you can determine which version:
//     ie === 7; // IE7
// Thus, to detect IE:
//     if (ie) {}
// And to detect the version:
//     ie === 6 // IE6
//     ie> 7 // IE8, IE9 ...
//     ie <9 // Anything less than IE9
// ----------------------------------------------------------
var ie = (function(){
    var undef, v = 3, div = document.createElement('div');
   
    while (
        div.innerHTML = '<!--[if gt IE '+(++v)+']><i></i><![endif]-->',
        div.getElementsByTagName('i')[0]
    );
   
    return v> 4 ? v : undef;
}());

more here

http://ajaxian.com/archives/attack-of-the-ie-conditional-comment