So far unfortunately we have been able to identify any specific data returned by the service that might be upsetting the iPhone sync client. The best workaround we have is to use 'Settings\General\Reset\Reset All Settings' to clear the device sync state. This will also reset various other setting such as your icon positions and stock tickers. Most users who have used this method report success, although some have reported the freezing subsequently returns.
If you have had freezing return after device reset, please send us e-mail and mention 'iPhone Freezing Returned' in the subject so we can identify your message quickly. We want to check the server logs for those users to see if we can correlate us sending a specific event to their device with the onset of freezing.
Also, if anyone has the freezing problem and is also an iPhone developer, we'd be interested to see the log from your device (can be viewed using the XCode utility with the device docked).