I've seen new adsense-CSS several times today:
http://www.ice-blog.de/wp-content/uploads/2007/02/adsense.jpg
or is my Wordpress hacking the original css?

No, it's just a test made by AdSense on some ads ;)

What Tom says... you can't affect the AdSense with your own CSS because it's loaded in an iframe on a separate domain. So design-wise you're restricted to what Google allows you to tweak at the moment...

that's what I thought... I'd bet on testing, though...

In a somewhat related note, I saw this AdSense code on a site (sensitive data removed). Does anyone know why does it look like this?

google_ad_client = "pub-XXXXXXXXX";
google_ad_output = "textlink";
google_ad_format = "ref_text";
google_cpa_choice = "YYYYYYY";
google_ad_channel = "ZZZZZZZZ";

>> Does anyone know why does it look like this?

What's odd about that? That's the same format code I get generated when I choose a referral text link.