I get a lot of scam emails and sometimes click on the links in them just for the fun of it. To see if the offending site has been taken down yet. To see how "well" they are done (recently it's really impossible to distinguish a phishing site from a real one by layout, the scammers have got good).
So I clicked on a random PayPal scam again when this popped up. This is the first time that I saw this warning. I think it is a very effective security measure as it blocks newbies from proceeding any further and sends a very clear message. I'm not fully sure, but I think this comes from FF's recent version of Google Toolbar that I have installed.
The only sad thing here being that I took that shot above while jerking around with window sizes to capture the complete forgery warning bubble before it could reposition itself. In its "natural" state, the bubble was as follows, which is a shame since the "report" link is hidden away.