Exchange Cached Mode (Exchange 2010) with Outlook 2003 and Outlook 2010 Causing Sync Issues

ad1) Please be more specific with sync issues. Do messages disappear? Do messages get duplicated?...

ad2) The basic issue is that Outlook 2003 support UDP and polling notifications. Exchange 2007 supports UDP, polling and Async notifications. Exchange 2010 only supports polling and Async notifications. This means when Outlook 2003 move from Exchange 2007 to Exchange 2010, Outlook clients will fall back to polling which by default only gets notifications every 30secs-1min. This means any change won’t show up immediately.

UDP notification support was removed from Exchange 2010. As a result, Outlook 2003 can only use polling notifications in online mode, which are still supported by RPC Client Access. This will result in a slight delay in updates to item status (30 seconds on average up to a 1 minute delay) when changes are made to items in a mailbox accessed by Outlook 2003.

source: http://social.technet.microsoft.com/Forums/en/exchange2010/thread/d5a610d5-b3cd-40a5-8653-634f3b174f4e

See this newly released fix for Exchange 2010: UDP Notification Support Re-added to Exchange 2010. It should help.