Looks like some kind of code insertion.
There's been a lot of attacks via FTP recently. It's probably 'decodes' to a set of links to dodgy websites that sell things, rather than anything particularly nasty.
But, get rid of it asap. In fact I would look through EVERY php, javascript and html file to see if they have been messed around with too.
After that, change your FTP password on a regular basis and use as many non-alpha, non-numeric characters as you can.
Paul.
PS I maintain a batch of sites and regularly use Firefox+HttpFox to quickly see what files/images/scripts are being pulled in.
PSS Go back and edit your post to cut that string into pieces - very hard to read the thread otherwise.