The iTrip plugs into the 30pin connection at the bottom of your iPod so you are unable to charge your iPod with it plugged in - even if you own a car charger. It should also be noted that the iTrip will reduce your iPods battery life as it does not have its own battery.
Before purchasing you should read about iPod car and iPhone car connection options and understand that this is an FM adapter:
iPod Car Connection Information:
http://www.icarkits.com/ipod-car.php
iPhone Car Connection Information:
http://www.icarkits.com/iphone_products.php