Instapaper iPhone app
Like other read-it-later type bookmarking tools, Instapaper lets you use a bookmarklet to flag articles that you'd like to read later, then gives you a web-based interface to go back and find those articles. The Instapaper iPhone app goes one further - it syncrhronizes offline copies of your saved articles so that you can read them even when you're not online.
Instapaper comes in both free and pro versions on the iPhone. The free version has met my needs perfectly so far, but I imagine if I was to go on a long flight, I would pony up the $10 for the full version,
[Update] Marco, the developer of Instapaper for the iPhone, chimed in in the comments to let me know that I was mistaken about the 10 article limit. I just re-tested on my copy, and sure enough there are 24 articles in there right now. My apologies, and thanks for letting me know, Marco!