Be aware that the Arduino Uno has a significant difference with older Arduino models, in particular, the Uno has a different serial comm chip so it is no longer the tried-and-true FTDI USB-serial chip and associated driver. I don't know what the issues are with the new Atmega8U2 or 16U2 driver, but that may be related to the difficulty (or not, if someone else already has an Uno working ok with the Pi). (I don't have either a Pi or an Uno so I can't test this myself.)
The Uno differs from all preceding boards in that it does not use the FTDI USB-to-serial driver chip. Instead, it features the Atmega16U2 (Atmega8U2 up to version R2) programmed as a USB-to-serial converter.
Revision 2 of the Uno board has a resistor pulling the 8U2 HWB line to ground, making it easier to put into DFU mode.
Revision 3 of the board has the following new features:
1.0 pinout: added SDA and SCL pins that are near to the AREF pin and two other new pins placed near to the RESET pin, the IOREF that allow the shields to adapt to the voltage provided from the board. In future, shields will be compatible both with the board that use the AVR, which operate with 5V and with the Arduino Due that operate with 3.3V. The second one is a not connected pin, that is reserved for future purposes.
Stronger RESET circuit.
Atmega 16U2 replace the 8U2.