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 which calendar serves as your default. A six-calendar sync took a noticeable amount of time, but I haven't seen any duplicates or other annoying sync problems so far (Edit: But you might want to stick with download-only at first, especially if you're already syncing through iTunes). 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.