Thanks to all who responded. Here is a summary of what I learned for the archives.

To combine two photos of the same scene shot at different exposures to increase
the apparent dynamic range, as described in technique #2 ("The Layer Mask")
in this Luminous Landscape article:

http://www.luminous-landscape.com/tutorials/digital-blending.shtml

the one tricky step for me was how to use one of the exposures, E2, as a layer mask for the other exposure, E1. Scott Bicknell described how to do that:

"What you need to do is create the layer mask on the target layer where you want to paste E2 . Then copy E2. Be sure no layers above your target layer are visible. Make sure the mask is active by clicking on it in the layers dialog. Then paste the copied image. It will become a floating selection. Then anchor the floating selection. That will merge it into the layer mask."

I also found that JD Smith has created a script-fu called exposure-blend that uses three images (light, dark, normal) to achieve the same effect. The script, including how to set its various input parameters, is available here:

http://turtle.as.arizona.edu/jdsmith/exposure_blend.php

As a new GIMP user I really appreciate this list--it is an excellent resource!

Josh Simons