http:// www.google.com/pagead/iclk?sa=l&ai=GZXaUBf
&num=42328&adurl=http:// offar.com?bZKvy

Google should use some hash with the URL (just like linkedin) to disallow direct URL rewriting

I tried the same trick by just changing the URL at the end & its working.
http://www.google.com/pagead/iclk?sa=l&ai=GZXaUBf&num=42328&adurl=http://blogoscoped.com

[Unlinked first URL – Tony]

Rohit, you could not believe how much URL are possible "flaw" like that :-S
But I don't often see this kind of flaw in my spams. Do you?