It works. Save this piece of text to a html file & check
<html>
<body>
<div style="position:fixed; left:20px; top:20px; background-color:#ff4;">
Fixed element<br/>
</div>
<div style="height:1000px;"></div>
</body>
</html>
I've checked this in Firefox 3.6.2
However, it will not work in IE6. IE 7 & 8 supports it only if you have turned on standards compliant mode (when you include the DOCTYPE ) like
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">