Bidirectional communication between Arduino and Raspberry Pi over USB possible? Is it possible to have bidirectional communication betweeen an Arduino Uno Rev 3 and a Raspberry Pi Modell B+ running Rasbian? They're connected ofer USB, and it would be good if I could use Python on the Pi.
Yes.  The Arduino Uno implements a serial protocol inside the USB signal using hardware.  This is how it is built.  The Linux side is a bit more complex as there are many USB protocols.  Here is a howto which talks about the Linux side of the Arduino / Linux connection.  

To be clear, Rasbian is a derivative of Linux.