So Google now provides a tool to allow you to pick the calendars that sync to your iPhone. It is:
https://www.google.com/calendar/iphoneselect
Once you’ve added your main Exchange gmail account, use this link to pick which calendars sync. I just saw this link today via lifehacker.com and have not yet tried it, but it looks a lot easier than my previous post.
The Intentional Geek 