Unfortunately, there's something awry with the way the iPhone's geotagged photos are being handled in OS X. At the moment, whether you import photos through iPhoto, Preview or using Image Capture, at some point the GPS reference may get set to North/West. This is a problem if you don't live in the northwestern hemisphere.
Regardless of whether Apple fixes this, you're still going to have to retag all of your old photos. So tonight, I wrote an AppleScript that automates the process. This script can either be set as the Automatic Task in the ImageCapture utility, or you can drag files onto the script for processing.
Copy and paste the code below into Script Editor, customize for your location, and then save as an application (eg. File Format: Application). I've #commented the code to help you find where to tweak the North/South/East/West variables.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080821073713119