I was lucky. They just wanted any proof of purchase, asked me what it was (I said telephone) and I had to open the package for them. The guy just said "Ah, a smartphone" and went to do the necessary paperwork. I didn't even have to show him the actual phone. I paid 28.67 Euros and was out of there.
Now I just have to get some toilet paper and can go home to play with my new toy.
The German customs sends my phone back because the CE sign is missing. I explained that it was bought only for development purposes in a small company, but as the shipping address was a private one, this argument was rejected, too.
Just a tip: if you can select the officer (gender), chose a male officer. My experience of border control (and i have a LOT, especially CH<->DE, CH<->AT, AT<->DE), woman control much more detail than men do.
My phone was sent back by German customs as well due to the missing CE sign and german manual. It seems the CE sign alone wouldn't have helped, because according to the officer it is also mandatory to have a german manual included.