The iPhone 3G client for InstaMapper will be available as a free application at Apple Store, soon. Email your 40-digit device identifier to email@example.com if you would like to be one of the beta testers. To locate your device ID, connect your iPhone, open iTunes, then click on the words "Serial number:".
Update 7/31: the beta program is now closed. Thanks to everyone who participated. InstaMapper should be available in the App Store within a few days.