If you want your iPhone app to open Google Maps and show directions from the current location, you need to set "Current Location" as the source address. However the exact wording depends on the language of the device.
Based on: http://www.martip.net/blog/localized-current-location-string-for-iphone-apps
Obj-C source from https://github.com/Sjors/currentLocationStringForCurrentLanguage (Also from original blog post)