Currently Apple doesn't allow third party software to edit the calendar data on the iPhone. This would then only work through a sync from Google Calendar to Outlook to iPhone. (if not Jailbroken)

If you jailbreak the iPhone (which is not recommended) there are other solutions;
- http://www.appleiphoneschool.com/2008/05/16/nemussync-053-google-calendar-iphone-sync-wirelessly/

Or use Google;
http://www.google.nl/search?q=sync+google+calendar+with+iphone&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

If you can live with RTM -> Mac -> iPhone, then BusySync will do it for you. It works for me and my iPod.

http://www.busymac.com/