I've got an unbelievable number of emails about this, but Google yesterday released its own (free) Google Calendar Sync tool, which provides one- and two-way sync between Google Calendar and Microsoft Outlook:

Sync your Google Calendar with your Microsoft Outlook™ calendar

Access your calendar however and whenever you want. Check your Microsoft Outlook events on the go with Google Calendar. View your Google Calendar information offline through Microsoft Outlook calendar. Learn More

This is a huge release for various reasons, but especially for those who use both Google Calendar and the iPhone (which currently only syncs with Outlook on the PC). Looking good.

Thanks again to everyone who wrote in about this.