Yea, actually I thought a bit more about it as well, and any real (analog) circuit will have a frequency response that tapers of to zero as the frequency goes t

Yea, actually I thought a bit more about it as well, and any real (analog) circuit will have a frequency response that tapers of to zero as the frequency goes to infinity. So that basically determines a Nyquist frequency for any analog circuit, even if that frequency is so high that it has no practical implications.