Penn happens to sport an iPhone, and like any modern cell phone, it has "airplane mode." I'm going to show you how to set the iPhone to this "no call" mode, and explain why you probably don't have to while driving. Plus, if you absolutely must talk while driving I'll explain a helpful trick with the 3GS. You should never under any circumstances try to type while driving -- pull over, already.
To turn off your iPhone's cell radio, thus preventing distracting text messages, push messages and phone calls, go to Settings > Airplane Mode and turn the switch ON. Pretty easy, yes, I know. But unless you are going to use the iPhone's audio in your car (and there's like a zillion apps for that), you could just as easily turn off the ringer. The switch above the volume buttons on the left of the screen controls the mute for external sounds. When you arrive at your destination, check your phone for missed calls, etc.
If you have a 3GS, you can use the Apple headphones that came with it to activate Voice Control, giving you a relatively safe method of interacting with your iPhone while driving (except for a proper, Bluetooth speaker/mic or in-car solution, I guess). Plug in the headphones and put the headphone directly connected to the microphone in your ear. Press and hold the "middle" button (in-between the volume buttons) for about 3 seconds, until you hear the beeps. Voice Control is now on! You can control the iPod functions and make calls, but I recommend you try Voice Control before you start driving. Note that there are voice dialing apps for previous iPhones, but I feel having to see the screen to push even one button is an unacceptable distraction.
The New York Times and many other news outlets have reported on the unreleased government study showing that even talking on the phone increased the likelihood of having an accident -- whether you are hands-free calling or not. Driving and chatting with a disembodied voice can be distracting, apparently. So, again, the only truly safe thing to do is to switch off your cell radio entirely (Airplane mode). This won't allow you to play streaming radio stations, but it may save your life. Otherwise, use your willpower to ignore the chimes, rings and buzzes, and if you absolutely must talk or text, pull over.