I've received my first set of AtmegaRFA1s for prototypes from Digikey. I've gotten one working to the point where I can talk to the chip via ISP; from here it's a short path to getting the bootloader up and running. I have run into one small snag with the current version of the Arduino environment (0018) -- it uses a version of avr-gcc that does not support the Atmega128RFA1.
Luckily, the Arduino team just used an old WinAVR install for the compiler tools and libraries. Dropping in a copy of WinAVR-20100110 into hardware/tools/avr fixed that problem. I still have to finish porting enough of the libraries to enable me to compile sketches, but overall the effort is going well.