I am going to give this app a try soon. Requires jailbroken iPhone.
Sync your iPhone calendar to multiple Google Calendar listings with NemusSync, a free application for jailbroken iPhones and iPod touches. While other sync utilities have been floating around for a bit, NemusSync doesn't require GooSync or another go-between app to function, and you choose whether the sync is one- or two-way, and whether it's automatic or launched when you tap it. A six-calendar sync can take a noticeable amount of time, but I haven't seen any duplicates or other annoying sync problems so far. The developer plans to implement alerts and reminders in future versions; for now, you can use GCal's email reminders or something like IntelliScreen. NemusSync is a free download for jailbroken iPhones and iPod touches, and is available in Installer.app's pre-configured repositories. [From IPhone: NemusSync Syncs Google and iPhone Calendars Wirelessly]