The turtle wants 0-5V (TTL) serial communications. The AVR butterfly has a level shifter that brings it's UART to RS-232 levels. A simple jumper between the butterfly and turtle would have been nice. Instead I have to look into using the turtles level shifting DIN connector.
But wait! According to this I can use the USI pins to emulate UART. This would be nice. Just 3 plain jumpers between the butterfly and turtle and a lot of tricky software ;-)