You can now integrate Google Calendar with iCal on the Mac, with Outlook on a PC and with the Objective-C iPhone and iTouch platform.  The iCal and iPhone/iTouch are updated instantly, with the PC version syncs at a user-definable time interval.

iCal

In iCal, go to iCal in the menu bar ->Preferences -> Accounts

+ to create a new account

Username: Google Accounts full email address

Password: Google Accounts password

Account URL: https://www.google.com/calendar/dav/<your google accounts email address>/user

Windows

Requirements: XP or Vista (32 bit only), Outlook 2003 or 2007

Download the installer:

http://dl.google.com/googlecalendarsync/GoogleCalendarSync_Installer.exe

Fill out the fields with your Google Account information.

iPhone/ iTouch

Requirements: Firmware 3.0 or above

Settings ->Mail, Contacts, Calendars -> Add Account… -> Other -> Add CalDAV Account

Server: Google.com

User Name: your full Google Accounts eMail address

Password: your Google Accounts password

Description: Name the account whatever you like

You are now ready to push events both ways

Additionally, if you want to sync both calendars and contacts with your Google account, you can use Microsoft Exchange by following the instructions here: http://www.google.com/support/mobile/bin/answer.py?hl=en&answer=138740.  This willow allow you to sync multiple calendars (up to 25 with firmware 3.0)

References:

Windows http://www.google.com/support/calendar/bin/answer.py?answer=98565

iPhone/ iTouch http://www.google.com/support/mobile/bin/answer.py?hl=en&answer=151674

Apple https://www.google.com/support/calendar/bin/answer.py?answer=99358#ical