But it struck me... why power the ATTiny with 5 volts? I went to the trouble of adding a 3.3 volt LDO to the thing... Why not have that power the controller as well as the target (if selected)?
If you do that, you can also dispense with the level shifting hardware for the USB pins - you can just connect them directly up! Because of the 74HC125 bus buffer, you can program 5 volt systems just fine, since it will happily level-shift the SPI signals up to 5 volts. When the bus buffer is powered by 1.8 volts, the logic high threshold for a 3.3 volt 2313 is still met, and the reverse is true when the bus buffer is powered by 5 volts.