Somehow my FTP password was hacked and this bit of JavaScript was inserted into all of the index.php files that run this site!
I found out through this email from Google – glad they told me as otherwise I may not have known!
Apparently this hack is a virus which uses stolen FTP password information then injects this bit of code to pages on the server without you realising it. Where they got my password from I’m not sure, though I suspect it might have been my old work machine. :reallypissed: