The iPhone lacks a built-in voice dialing mechanism. However, it does offer easy access to AT&T's $5-per-month VoiceDial ("VoiceConnect") function, which is accessible by tapping "Settings," then "Phone," then "AT&T Services," and finally "Voice Connect," or by simply dialing *08 on the keypad.
The service, unfortunately, requires that you set up a remotely stored address book (it can't dial the numbers stored on your phone). You can, however, import contacts from Yahoo!, Outlook, and other email programs. It stores up to 2000 contacts.
VoiceDial also requires more steps than most built-in voice dialing functions. You have to either press the phone button, then the keypad button, then dial *08, or store *08 as a contact.