From one of the readme files...
So a copy of the Arduino 18 core is included here.
The problems are because optiboot includes an old core #. I suggest staying away from that version of optiboot.
Use the optiboot version westfw has graciously provided...
http://arduino.cc/forum/index.php/topic,64105.0.html
# I will refrain from making harsh sarcastic comments about why bundling a core with a new bootloader is a bad idea. And why abandoning the download site is cruel.