Yeah, I know, this subject has been beaten to death a million times already.
I've searched the forums, and I haven't found anything quite as strange as this.
Ok, so I build a PC board and ISP'ed my program to an ATmega16 with the STK500.... tested the board, everything works fine. Now, several weeks later, I want to change the program on the chip. Can't enter programming mode. However, the current code in the chip runs just fine when I apply power to the board, the STK500 is fine (I've programmed other chips with it).... physically the board is perfect, all traces intact, etc.
Is there anything else that could be messed up, or am I going to have to assume the ISP circuitry in the chip died somehow and replace it?