wrote:

That notwithstanding, I'm curious about reciprocity with digital
sensors, if only to know what is going on a bit better. Is there a
point at which doubling the exposure time doesn't produce a "one stop"
change, as with film? Or makes a color change?

Well, here are a couple examples:

http://www.pbase.com/image/34152731

"This is a long (almost 9 minutes) exposure that I took last night using only
the light from the moon." - Annika1980 (circa Sept 2004)

Here's one I took under moonlight as well:
http://www.photosig.com/go/photos/vi...KI2?id=1716559

I also did a litle experimenting with the ISO settings but not in a
systematic way, I'm afraid. Assuming no camera shake, and a stationary
subject (buildings, etc.), would you use a lower ISO and longer
exposure or the other way around? I'm wondering which results in less
noise.

Lower ISO will generally mean less noise, but remember it also needs a wider
aperture for the same exposure time, which will lose some DOF, and that a longer
exposure will eat up more batteries.