I have a Cingular (HTC) 8125 that runs windows Mobile 5. I use Gmail and Google Calendar. Mail works fine on the phone, but there's no way to read by Google Calendar.
A couple of options have popped up. The most interesting is an Open Source program that syncs Outlook with Google Calendar. You can then sync your phone/pda with Outlook. Round about, but useful. Unfortunately, it's a little complicated to set up.
To get the sync program, which is called Remote Calendars, go to the download page on Source Forge. The latest msi is what you want. But wait!
First, you have to prep Windows and Outlook to run the program. How?
Follow these directions. They look complicated. Heck, they are complicated, but they're straightforward and they work.
The final step is to make sure the sync is going both ways: from Gcal to Outlook and Outlook to Gcal.
When RemoteCalendars works, it's great, but I've had a problem. The RemoteCalendars tool bar options aren't always available and sometimes the toolbar itself disappears. Other people have had the same problem and found solutions.
Be careful, though, they involved editing your Windows registry. I've done it a lot and never had a problem, but for some people it's led to disasters. The other issue is that one of the fixes seems to disable an Outlook security feature. BE CAREFUL!
GCalSync seems like a perfect solution, for someone else. It won't work for me because it doesn't work with Windows Mobile, yet. But there's hope. It's Open Source and that means a lot of smart, public-spirited people are working ti improve it and it will consistently get better.