Hi all. The good news is that it looks like a solution exists with the iPhone 3.0 software, which will be generally available next week...
3.0 supports CalDAV, and as such you can subscribe directly to any single Google calendar that you normally can access, with full writability from the native calendar app.
Only hitch is that it doesn't give you delegates "for free" like the desktop iCal does. Instead, you have to set up a subscription for each one individually supplying your google username/password and the URL:
https://www.google.com/calendar/dav/<CALENDER ID>/user
... where <CALENDAR ID> is a long unreadable mishmesh which is taken from the calendar's Details>Settings>Address field
Fortunately, 3.0 also provides copy/paste, so you can work out the long URL on your desktop, and mail it to yourself for pasting into the field.
Cheers!