In looking over the AVRISP help document I found the following
RESET Used as General IO port.
If the RESET pin is used as a general purpose IO, In-System Programming is not possible. The reason is that the RESET pin must pulled to 12V to enter High Voltage Serial or Parallel Programming mode (HVSP or HVP). HVSP or HVP must be used to change the RSTDSBL fuse.
All the data sheets I have read only talk about low voltage serial programming, which cannot be used if the RESET is used for general I/O.
Where do I find more info on HVSP?