Although I've lived my entire geek life in PC-land, I am a big fan of the iPhone. The lectures on iTunes U are in my opinion enough to justify the two-year additional cost of ownership. Having said that, I don't think the iPhone is an essential tool or even a particularly useful tool for chess players. (They do make cheating on the toilet easier, so perhaps they are already de rigueur among a certain subset of World Open participants.) But if you've already drunk the Apple Kool-Aid, there are a couple chess apps that are nice to have. I thought I'd tell you about the ones I have.
ChessClock (Samuel Kass) is one of several chess timer iPhone apps. It's not really suitable for tournament play. No iPhone app ever could be, as the touch screen doesn't give you tactile feedback (Nothing is more annoying than double-checking that you successfully stopped your clock in a time scramble.) But you can put the iPhone in airplane mode, turn off the volume, and use it as an emergency tourament clock. I've done this a couple times in the past year: it's acceptable for that limited purpose.
The clock is also the clock button!
There's also an "analog" mode: cute, but not very useful.
I play chess, so I'm not totally dense. But I'm also one of those over-50 people who has delegated control of home entertainment systems to the teenager in the house, and I am intimidated by the non-intuitive methods of setting certain chess clocks. ChessClock shines in this regard: it supports Fischer, Bronstein, and "USCF" (andante) modes (Bill Smythe explains the differences here) for any increment between 1 and 59 seconds.
There is no handicap option for giving one player a larger increment, but one can easily give one of the players more time in the main time control. The settings are simple and intuitive.
You can only set the clock to the nearest minute, but the digital display gives you the nearest tenth of a second. Strange.
The price is right at $2.99: it's fine for casual rapid play (or family Scrabble games), and when the iPhone is fully charged, it can be used as an emergency substitute for a round of tournament play. It won't work for blitz play because of the tactile feedback problem. But if you use the app once, it pays for itself. Recommended.