So, it seems that Google now has CalDAV support, which means that Apple’s iCal application can natively sync two-way with Google Calendar without any (sorry Spanning Sync!) third party applications!
CalDAV setup instructions for iCal
Once you’ve got that setup, and you can use it with calendars others have shared with you (and they seem to follow the same permissions perfectly), you can simply modify the calendars in iCal.
These calendars will sync beautifully with the iPhone iCal application, however you cannot make modifications to them from the iPhone. I did notice that when adding something using the iPhone gCal interface it very quickly appeared on my desktop iCal. So I might just stick with gCal/iPhone+iCal. It’s not a bad solution for free