Forgotting for a while Arduino, can I use a stand-alone Atmega328 to program another stand-alone Atmega328?
Strange question, isn't it? 
If I have 1 Atmega328 on a board with an ICSP conn, if I connect to it another board, can I program the first Atmega using only a uC?
Maybe using RX/TX or I2C, I don't know....
Sure. Load ArduinoISP into the "programmer" 328, and it will program the "target" 328 just fine.
There's no reason that a "stand alone 328" can't run Arduino Sketches.
Guys, I was wondering something similar. Id like to burn Arduino sketches directly from an arduino-as-AVRISP to an unbootloaded 328 using the ISP connectors. Do I need to change the preferences.txt file to read:
upload.using = atmega328
or is this handled by the arduinoISP program installed on the programmer?
in preferences.txt,
upload.using = arduinoisp
will do the trick