I ordered an iPad the other day. It's not here yet, but it's not far off either. One of the reasons I bought it was to save me the trouble of carting gaming books around. Since most of the games I play also come with a PDF, a decent PDF viewer will help out no end. I hear that GoodReader is the way to go for this, so I'm sure I'll get it and cram every gaming PDF (along with lots more besides) into the iPad. It won't replace my interest in having the game book as a physical artefact, or even as the primary source for learning the rules. Rather, it'll be useful as a supplement to the book. A searchable PDF is more useful than a sparse index any day.
But, ah hah! This is a workaround to a larger problem: having the relevant information close to hand so that the story is uninterrupted. Most of my gaming is convention gaming, introducing people to new games. I have three hours to introduce them to a game, including explaining the rules and playing out a scenario. Even with the speedy resources of an iPad at my disposal, it's still a distraction from the fiction and a disruption to flow of the game.
So I'll still prepare my cheat sheets, the one-page summaries of the rules for new players, and hand them out (I may even save them as PDF for my iPad). These are more useful than you know. The only thing more useful than a cheat sheet, is a cheat sheet that is part of the character sheet. Giving a new player a single page, complete with their character and the relevant rules is the best supportive artefact I can give in that context. It keeps the focus on the game, not on the game book (paper or virtual) and allows us to fit more game into our day.