westfw:
I don't think you can use A6/A7 as digital pins AT ALL. They are only connected to the analog mux.
Oops! You're right. The ADC6 and ADC7 pins have no other purpose. They are not part of any I/O register. PORTC doesn't have a BIT 7 and BIT 6 is connected to the Reset pin if the RSTDISBL fuse is set (Reset is disabled). If the Reset pin is not disabled, writing to that bit will do nothing and reading it will always return 0.