Oh Just to avoid confusion, I did at first use the USB-cable that I got with the iPad 2.
And when that didnt work, I tried two different USB-cables for iPhone 4 too. But it was the same error in iTunes.
I just thought if it was something wrong with the cable for the iPad 2, maybe some other cable would work better.
But I donno, it should work, because its only a few weeks old.