Yes, it looks like you’ve been compromised. If you edit the css template, do you see the added code there? I suspect not- and as silenz recommends, check your index.php, path.php and config.php for added code. If you aren’t sure what you’re looking for, let me know and I’ll walk you through it.
Security is top priority for Ellislab, so let’s get the site fixed and pursue how this happened.
To help us with that, can you let me know if you’re running any other scripts? And are you on shared hosting? Also, please report this to your host immediately as they can help identify where the attack originated from so that steps can be taken to prevent this in the future.