Unlike many people, I've been getting the long-form Google URLs almost exclusively for a long time. So if I search on "google" and hover on the headline, in my status bar, it shows a URL like:
http://www.google.com/url?sa=t&source=web&ct=res&cd=4&url=http%3A%2F%2Fwww.google.com
%2F&ei=OewWSsXFFKTgtAOJjdiSDw&usg=AFQjCNG5-9Jej-ukVeakTgwonqt2narbYg&sig2=4Lwo00y104At7P9SCT7uXA

I use the "My Library" feature of Google books a lot, so I'm often signed into a Google account. I'm not sure if that matters. I do have "search history" turned off and all that, but I suspect being logged in changes things.

Anyway, lately I've been noticing more URLs that are actually just the destination. So for the "google" search, I hover on the link, and it's just like it used to be: http://www.google.com which matches the link given at the bottom of the result.

Now here's where it gets interesting: if you CLICK on the link, it is transformed into the long form. Normally if you just click, you wouldn't see this and wouldn't know that Google is using Javascript to rewrite the URL *client* side *before* submitting the GET request.

The way I noticed is that if you *right* click, it fires the event and the URL gets transformed. So I haven't left the page and when I hover over a link that I have right clicked, I now get the long form in the status bar. In other words, there's some DOM scripting that is actually changing the URL.

A little look at the source code show that it fires on the OnMouseDown event.

So the URL looks like this:

<a href="http://www.google.com/" class=l onmousedown="return rwt(this,'','','res','4',
'AFQjCNG5-9Jej-ukVeakTgwonqt2narbYg','&amp;sig2=4Lwo00y104At7P9SCT7uXA')">

In the doc head, the rwt function is defined as:

window.rwt=function(a,d,e,g,h,f,i,j){
try {
if(a===window) {
a=window.event.srcElement;
while(a){
if(a.href)break;
a=a.parentNode
}
}
var b=encodeURIComponent¦¦escape,c=a.href.split("#"),k=["/url?sa=t\x26source\x3dweb",d?"&oi="+b
(d):"",e?"&cad="+b(e):"","&ct=",b(g¦¦"res"),"&cd=",b(h),"&url=",b(c[0]).replace(/\+/
g,"%2B"),"&ei=","3cgWSoiDAZmWswOEhvTZCA",f?"&usg="+f:"",i,c?"#"+c[1]:""].join("");

a.href=k;
a.onmousedown=""
}
catch(l){}
return true
};

[edited by: tedster at 7:12 pm (utc) on May 22, 2009]
[edit reason] added line breaks to prevent side-scroll [/edit]