you want to find the pixel width of a block of text if it was all one line?
I think what you might be able to do (and someone will downvote this for being wrong) is to use javascript to set up another span in another div off the page (e.g. style="top:-500px;"), take the 's innerHTML, drop it to the hidden span, take the width of the containing div, and then run with that.
I predict cross browser issues and heartache
<html>
<head>
<script>
function getSpanWidth(spanID){
sourceSpan=document.getElementById(spanID)
targetSpan=document.getElementById("spanmeasure")
targetSpan.innerHTML=sourceSpan.innerHTML
alert(targetSpan.offsetWidth+" "+sourceSpan.offsetWidth)
}
</script>
<style>
div{border:1px solid #cc0000;}
</style>
</head>
<body onLoad="getSpanWidth('spantext')">
<div style="width:100000px; top:-1000px;position:absolute;"><span id="spanmeasure"></span></div>
<div style="width:200px;">
<span id="spantext"> span text goes here, and then if there is lots and lots it will wrap around and be awesome! I'll put in more text to awesome it out even more... Awesome!
</span>
</div>
</body>
</html>
this seems to work on ie7 and ff. You will get variation if you don't use a reset style (because the default fonts are different)
also, if you set display:none, then the width will be 0.