I tried the TinyDebugSerial, same problem.
MontySylver:
I tried the same thing with an ATTiny85 and for some reason it does only work when VCC is not connected. I haven't tried it with an 84 yet, but I guess it's not that much different. Hope that helps.
What do you mean by not connecting VCC? Two different power supplies for the two microcontrollers? Because at the moment the AtTiny receives power from the Arduino 5V rail.