My favorite tool for offline reading on the iPhone is actually another browser. Before I go on a flight, I load up HN, open up links to stories and comment pages I want in iCabMobile, which then saves them for offline reading. I've found it be more effective than Instapaper because I only need to load it once, and I end up reading a copy of the webpage itself, with all formatting, images, layout, etc.

If Cyberspace can support a similar feature, I would gladly purchase it, because iCabMobile's offline support isn't as well supported on the iPad.