The iPhone mic is not disabled when headphones that don't have a mic are used (Apple's iPhone earbuds have an additional contact ring on the plug). If you have a favorite pair of earbuds from you old iPod, you can use them with your iPhone and take/make calls without unplugging. Simply take/make the call and place the iPhone a bit lower on your face and talk away.
While not quite as great as a fresh pair of $100 microphone-equipped earbuds, it works. Of course you may also need a $10 adapter to get your phones to work anyway. (Credit goes to my daughter for this one.)
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080212133420675