Darren over at digital-photography-school.com posted a tutorial that is if interest to RE photographers that are not using Photoshop CS3. The tutorial, called Stacking Images shot without a Tripod illustrated how to stack and align images using PTAssembler (shareware) or Hugin (free).
The problem is if you want to shoot several images so you can blend the sky from one image with the foreground of another image you need to have the two images perfectly registered (aligned pixel for pixel). There are several was to do this:
Of all of these I still like 1. the best. With this technique you can extract multiple exposures many stops apart in exposure from one RAW image. When shooting panoramas when this technique is invaluable.