Article here (in German).
Basically, the first connection to port 80 (http) or 443 (https) after opening a connection gets redirected to their portal instead—as "customer service". Woohoo.
They're testing this in a couple of regions in Germany and will then consider rolling it out nationwide.
That's something that makes me happy. If I type in http://www.example.com/ I want to be taken to http://www.example.com/ and not to http://portal.1und1.de/ or whereever, even if it's my first connect after "dialling up". Plus what will happen with things such as DynDNS scripts or antivirus updates trying to retrieve legitimate data automatically and being fed the portal page instead?
People in the comments page say, "Oh that's easy, just put lynx -dump http://randompage.com/ in your ip-up script" but (a) not everyone has Linux and (b) we shouldn't have to do that!
Meh. Maybe it's good that I didn't buy their flatrate yet (which would mean I'd have to give 12 months' notice again IIRC)... if they start doing funny tricks like that, I may think about switching providers, and my existing domain they host be darned.