I was wondering if I can use MISO, MOSI for both programming the device and also as I/O pins once programmed? I need the extra pins for a project, but also need the pins to program the device. I'm guessing I can, since when programming, the the other use of the pins would be disabled as long as reset is pulled down by the programmer, but wanted to make sure.
Thanks,
John
You can, as long as thay don't interfere with the ICSP or you're projects breaks if the pin is driven during ICSP. For example, a button to GND used with internal pull up is fine (as long as it's not pressed during programming) but a switch or sensor output (aka the pin is externally driven) it will screw with ICSP. And driving a led (with low current*) is fine but if that pin drives the launch or a nuclear missile you might not want to do that because it will get triggered during programming. 
*Because the ATtiny and programmer (depending on it's MISO or MOSI) needs to be able to drive it while programming as well.
Thanks septillion - that all makes perfect sense, pretty funny too about the missile. 