This post is for anyone using the iPhone with an existing (non-Google) calendar who also has Google Calendars they want to sync with.
iPhone’s new OS 3.1 includes CalDAV support. I don’t know all the details, but this feature allows you to add a Google Calendar to your iPhone calendar. Previously the iPhone allowed you to do this via iCal and Google Sync. Unfortunately, this method meant replacing your existing calendars with the Google Calendars (up to 25). I sync my calendar to my work account (Exchange) but wanted to add Google Calendar to the mix. With OS 3.1, you can now do this.
However, the “standard” approach only allows you to sync with your “default” Google calendar. If you have multiple calendars, it is a bit trickier to get your calendars added, but there is a way!