I just got a new and shiny iPhone, and I wanted to sync a few Google calendars to it. It took a bit of Googling to figure out how to do it, so here's the recipe.
First, check out this article to do the initial account setup for your Google account in your iPhone: Google Sync: Set Up Your Apple Device for Google Sync.
Then go to m.google.com/sync using Safari on your iPhone. There you'll probably get an error message saying that your native language is not supported — not very user friendly by Google. The solution is to scroll down to the bottom of the page, and click on the link to change language. Change it to English, and you should be able to sign in with your Google account.