The much lamented failure to include a voice dialer on the iPhone is cured with the free application, Say Who. After installing and loading you get a simple screen that asks you to hold a button and say a name or phone number.
It then works flawlessly. The only downside is the delay to initiate the microphone when you first turn it on (it takes about 5 seconds on my 1st gen iPhone) and the physical process of holding down the button while saying the name. This is certainly possible while driving, but not preferable. All things considered, this free application is a must have.