Found this on google code:
Comment 9 by logansanoh, Feb 03, 2010
Maybe needs to be added to the WIKI - I found that sync has issues if your time and date and regional settings differ between the phone and gmail/google online settings.
Also, (and I am going to have to put this in several open issues as its a wide spread issue) - going from WIFI back to 3G, when the wifi drops out, will kill the connect ability to google servers. Internet will still work but cannot connect to google. If you switch wifi off and back on when in range again, it all works fine. If you switch wifi off while in range back to 3G, it also still works. Current solution from google on this is to perform a HARD RESET.
Your welcome.
Issue 1866 - android - Sync autenthication problem - Project Hosting on Google Code
Comment 9 by logansanoh, Feb 03, 2010
Maybe needs to be added to the WIKI - I found that sync has issues if your time and date and regional settings differ between the phone and gmail/google online settings.
Also, (and I am going to have to put this in several open issues as its a wide spread issue) - going from WIFI back to 3G, when the wifi drops out, will kill the connect ability to google servers. Internet will still work but cannot connect to google. If you switch wifi off and back on when in range again, it all works fine. If you switch wifi off while in range back to 3G, it also still works. Current solution from google on this is to perform a HARD RESET.
Your welcome.
Issue 1866 - android - Sync autenthication problem - Project Hosting on Google Code