Folks at NuevaSync have come up with a great solution, which lets you sync your Google Calender enteries with your Windows Mobile device
NuevaSync allows direct, over-the-air, native synchronization of certain smart phones and PDA devices with public PIM, and calendaring services including Google Calendar. NuevaSync does not need any software installed on your device because it uses synchronization protocols that are already built in.
Please note that Windows Mobile 5 and onwards are supported. This means that Windows Mobile 2003 or older versions of Windows Mobile are not supported.