I have a synthesizer keyboard that outputs 0-5v depending on what key is pressed. Is it safe to connect that voltage directly to an adc pin or will I need to use an opto-isolator chip? Conversely, is it safe to connect the pwm output directly to a 0-5v cv input on the same keyboard? My nerdkit is powered by a different power supply than the keyboard.

5v is the typical power supply for the nerdkit, so it should be save to connect your keyboard to the ADC pin. However, make sure that 5v IS the maximum voltage from the keyboard. As far as I know, analog sound data usually goes from 0-1.4v, not 0-5v. It'd still be safe to plug it in, you just won't get any ADC readings higher then about 287.

Also, make sure that the ground of the keyboard is connected to the ground of the nerdkit too, otherwise you won't be able to read the voltages correctly.

Thanks. I measured the voltage coming out and it is indeed putting out 0-4.98v on my multimeter. I am trying to read a control voltage from a Roland SH101, so it is not audio.

is it safe to connect the pwm output directly to a 0-5v cv input?

PWM output isn't 0-5V, it's 0V or 5V, switching rapidly back and forth between them. If that's not what you want, you have to use a low-pass filter.