Note: this was before iOS5.
So you’ve got your initial calendar up and running on you iPhone as CalDav, but you want more… There’s no way to select one of the other calendars you’ve got access to from the settings menu. But there is a simple trick that involves getting the right url to add multiple CalDav calendars from the same Google Account. These calendars won’t be read-only, they are synced in two ways.
Nathan de Vries wrote it down. To my surprise there seems to be a very cool (enterprise) iPhone configuration utility.
If you’ve got questions, I’ll try to answer them. But read through the whole thing first.