Experience from Geert Langereis:
- Only the newest version of the Arduino Programming Environment is UNO-compatible (Version 0020)
- You no longer need the FTDI VCP USB drivers, instead you use default VCP on Mac OS-X and a different .inf file on a PC. When you plug in the board, Windows detects new hardware and you have to open the .inf file in the “drivers” directory of the Arduino software. This is also the only procedure to get the board running if you will use AVR-Studio instead of the Arduino PDE.
If you use AVR-Studio, you need a small modification in the AVRdude upload command line. It is either
avrdude -p atmega328p -c arduino -P com11 -b 115200 -F -U flash:w:%HEX_FILE%
or simply
avrdude -p atmega328p -c arduino -P com11 -F -U flash:w:%HEX_FILE%
What has changed is the baudrate. For me, both lines are working.