When using the clamp on programmer the chip can twist so that the pin contacts are intermittent. If avrdude suddenly starts reporting bizarre device signatures (or just 0x00000000 or 0xffffffff) check to make sure the chip is parallel with the clamp jaws.
The first step is getting the ATtiny25 to be a blinky.
In theory that looks something like this:
However, rather than the DIP8 format I have a surface mountable ATTINY25-20SU, so it's going to go in this clamping gizmo that I bought off ebay.
When the clamp tightened down it felt like the pins bent. Hopefully the chip will still be usable later.
Burning the bootloader worked perfectly and the chip should be running at 8Mhz. Modifying the Blink sample to toggle pins PB3 and PB4 seems to be working.