It seems to be working for me. I downloaded the 1.3 firmware, opened Single_Arduino_SNAP.pde which started up the Arduino "0011 Alpha" development environment. I was able to download the program to the Arduino Diecimila. I got a "Can't determine program size" error the first few times I verified (compiled) but after downloading to the Arduino that error did not repeat.
When I run the serial monitor at 19200 baud I see a "dBEGIN\n" message each time I reset the Arduino.
I think there was a step I had to do the first time I downloaded the RepRap firmware. The contents of the reprap-ardiuno-firmware_1.3/library directory must be copied to the arduino-0011/hardware/libraries directory that is part of the Arduino install.