I just had the same problem. It seems ALL the .js files in my installation were modified at 1:23 am this morning.
I learned this by doing a scan with sucuri.net
The following code was included at the bottom of each file.
[Code moderated as per the Forum Rules. Please use the pastebin]
I just downloaded all my wordpress installation… used notepad++ to run a search and replace in files …. then uploaded the whole thing again, overwriting all files from the server with the newly corrected files.
Sucuri.net re-scan.. and all clean..
This is a temporary solution as I am not sure where the hackers got in, or what caused the problem… so I am sure it will come up again..
If anyone else knows the source of the problem… please let us know!! ??
Hope this helps you all.